owikh84, let say only one worker failed or stopped during the prime test, what should I do?

increase RAM voltage?

I read, some said increase the ram voltage

problem is, my ram is 2400MHz and 1.2v

I'm using crucial ballistix sport lt 8gb and single

--edit

right now I'm running at 1.3v 5GHz on normal usage.

1.104v is not the worst chip as I've seen a 1.184v or more somewhere else before.
My 8086K is not better than my 8700K, it's doing 1.344v at 5.0GHz @ 6 cores but still better than average 8700K at 1.35-1.4v.
I've just sold it two days ago and bought a Ryzen 2700X at RM1299 for 2nd benching rig.
Last minute changing the plan HAHA.

To all 8700k users, which motherboard would you suggest aside from Asrock (i know they are good boards)?

Are you using any Asus Z370 board? 

Stock clock and no OC? 

Other than Prime95, you can also stress with LinX, OCCT, Realbench v2.56, AIDA64 etc. All this is listed in the Post #1.

For testing AVX stability, P95 (with AVX/FMA enabled) Custom Blend 1344K min/1344K max for 3hours+ is sufficient enough.
Small FFT will give you unrealistic load, so avoid using it unless your cooling is able to handle the heat output.

Also, watch out for the VRM temp (read via HWiNFO64 if your mobo has VRM temp sensor) as it will get hot very fast without active cooling facing toward the VRM heatsink.