A common problem with factory overclocked ASUS r9 280x.
Especially if your mode is http://www.asus.com/Graphics-Cards/R9280XD...specifications/

Asus uses GDDR5 rated for 1500mhz for the board, but overclocked it to 1600mhz.
To make it worst, https://www.techpowerup.com/reviews/ASUS/R9...u_II_TOP/4.html shows the VRAM chips arent actively cooled.

You only need to downclock the vram clock to solve it. No need to downclock gpu core clock.

I have a Gigabyte 280x as well on my 2nd PC and it also has the same problem during gaming.

It's definitely a VRAM issue. Lots of 280x have this problem.

I manage to minimize the issue by forcing the fans to run at higher speed at lower temperatures. Using MSI afterburner to do this.

Still contemplating whether to claim warranty for this.

If the VRM is faulty, your card shouldn't even work.
Can you check the VRM temperature?

Did you encountered any artifacts/graphic anomalies while gaming?

Hmm....if it is VRAM clock, wouldn't setting back to 1500Mhz (Hynix chip original frequency rating) will solve it for his gpu?

He ought to update his vbios first before playing around with vram and gpu clocks.

As Skidd said~

Now, faster update to the latest vbios and report back if the issue still appearing.

Why your card temperature is higher than my 290x at idle? You're on CFX?
Why you still on 15.5? Hop on 15.7 masterrace.

Btw your card default voltage is 1.25v?

Can you upload the vbios dump here? Open GPUZ, BIOS version ---> click the arrow---> save to file.

I wanna check if the asus vbios flasher actually update your gpu vbios or not.

EDIT:
Please check using msi afterburner for memory voltage, is it still 1.6V?
MSI afterburner ---> settings ---> tick "unlock voltage control" and "unlock voltage monitoring".
If it still 1.6V (1600mV), then you have failed to update the vbios.

........so, your model has the latest vbios which reduces the VRAM clock to 1.5V installed properly....

  and yet....the flickering still occurs....... According to you, reducing the VRAM clock somehow alleviate it....

We can't rule out the VRAM as source of flickering yet.

Hmm....Wanna try the opposite method?
Increase the VRAM voltage slightly to 1.52 or 1.55v.
Set the VRAM clock back to 1500Mhz.

My reasoning, 280x got 12 vram chips around the board. Perhaps the memory controller doesn't output 1.5V precisely for 12 VRAM chip.
Undervoltage also can cause flickering issue.

EDIT: Write too fast until need to correct so many times......

So....after you test with faster fan speed, does the flickering/artifacts still occur?

If it didn't occur, then it might be subpar hynix VRAM even though it is running at correct 1500mhz and 1.5V.

You can create a custom aggressive fan curve for faster fan speed.
MSI Afterburner --> Settings -->Fan -->Tick "Enable user defined software automatic fan control".
You should know how to set the curve  . Quite intuitive interface.

edit : Once you click OK, remember to HIGHLIGHT "User Define" near the Fan Speed (%)
Wait a minute.....does vram slight increase in voltage to 1.52 or 1.55v fix the artifacting at 1500mhz?
Or because of the max fan speed?
Which one work for your gpu? Both?

I am getting confused here...

Good thing you use witcher 3 to test. This game is really good in stressing GPU component especially in city of Novigrad at ultra settings.
1519mV for VRAM should be okay (never got the precise output anyway)
If there isn't anymore artifact after 1519mV and fan speed of 80% applied......then you got lemon Hynix VRAM which is too subpar (prone to instability and overheating even with the rated specification setting being applied).

Since you have been playing with fan speed at high rpm, why not apply a bit of overclock for your gpu core clock as well?
A little guidance ;
GPU core clock set to 1170Mhz (1225, 1188 or even lower 1160, for gpu core clock = the higher, the better) with gpu core voltage set to 1300mV (or 1265, for voltage, the lower the better).
Power Target set to 120% (aka +20%)
I only suggest this because you have been setting a high gpu fan speed at 80%.

The only downside of high fan speed = dust problem. Remember to keep fan and heatpipe fins clean.

Do more test with witcher 3.  (Technically, for stressing purpose, unigine or furmark tends to be used)

EDIT:
Remember to report back if the gpu core OC is stable and which value you use.
Maintain the vram clock at 1500mhz and vram voltage at 1520mV (1519). Fan speed at 80%.
Stress the GPU to 100% usage, check for any artifacting/crashing/blue screen and report back the maximum temperature.

If the GPU OC is stable, I got final testing for you to do.
By the way, you didn't tell us your power supply unit model?

It's normal to get lower or higher voltage that your setting, it's either Vdroop or some sort of voltage table the VRM controller must follow. And it is normal that sometimes lower voltage gets better result, especially on your situation where the VRAM doesn't have heatsink on. Try to lower fan speed on the same setting you did to see whether is it because the VRAM temperature is the culprit is the problem or not.

Don't follow JohnLai suggestion. When you know there is problem overclocking it further is NOT the solution (increasing power target is fine), also if the VRAM really 'lemon' it will cause artifacts no matter what the voltage.

Good PSU, can rule out unstable pcie voltage supply.

Well..... the tweaking is up to you.

In case if you RMA your GPU, remember to double check the VBIOS version once you got a new one.
Later you got the batch with 1.6V + 1600Mhz vram = sure the vram dies sooner.
So, update accordingly before heavy gaming.

Eh,so how ar? You said you will test the gpu for more than 3 hours, still got artifacts?
EDIT: If only set the VRAM 1519mV with fan speed auto, still artifacting? Or fan speed 80% is needed?

Might as well as increase it to 1600Mhz and see if the artifacting still there. (Fan 70%, shouldn't have any issue with temperature)
There were reports on VRAM becoming unstable at low clock speed. Just dont change the VRAM voltage. Maintain it at 1519mV.
Some VRAM chips are weird in a sense the chip becomes more stable at higher clock than lower clock. Generally, the lower the speed, the more stable.
-.- flicker when browsing sites? What kind of flicker? Horizontal small lines flickering? Checker board pattern flickering? Large vertical line flickering?

Okay.....this is probably 2D application VRAM downclocking issue.

Hmm......not sure if msi afterburner got this functionality, I only know ASUS GPU Tweak can set the minimum vram clock speed for low GPU load (2D load).

Need to set minimum 2D VRAM clock of 300mhz or 600mhz (preferably 300mhz)
Cause by default 2D VRAM clock speed is 150mhz (some said this frequency is too low for the vram chip to function correctly or the memory controller can't response fast enough to supply voltage for the operation)

When the gpu memory controller steps up vram speed from 150mhz to 600mhz or even 1500mhz under load, the flickering occurs.
You could use asus gputweak, it has similar controls as msi afterburner.

The setting you have done in msi afterburner only applies for 3D heavy application (under heavy graphic load). 

[attachmentid=4643345]

From the gputweak manual, page 18 and 19. Need to change to advanced mode for the 2D/3D option to be available.