AVX offset has been a debatable issue since Kaby Lake. Some said it's a bug while others saying otherwise.
It could be that you have certain OS software running in the background that's using AVX instructions, triggering the AVX offset thingy. Here's the list of AVX-based software: https://forums.anandtech.com/threads/what-c...d-avx2.2498660/
Some games might be using AVX instructions, or NVIDIA driver itself is implementing it: http://www.overclock.net/t/1641867/avx-offset-bug
This doesn't happen with Skylake-X though or he needs more games to test: http://www.overclock.net/t/1643198/coffee-...ks-while-gaming
One guy said this bug is triggered when C-states are enabled in the UEFI: http://forum.gigabyte.us/thread/2704/psa-avx-offset-bug-real
I personally don't use AVX offset because I want my OC to be all out for worst case scenarios. 
Check out the OC guide in the top post of this thread for GIGABYTE's Z370 GAMING 7. It will guide you OCing your 8700K to 5GHz.

But I don't think you can comfortably run 5GHz OC with your current air cooling, unless you hit silicon lottery with a really low VID (stock voltage with everything defaults) and delid. So it really depends on good is your CPU which varies from one chip to another. Get a bigger and better cooler like the NH-D15/S or 240mm+ AIO.

It is advisable to check your VID through the UEFI first before proceeding for OC. Check your CPU core temps with your CPU running at stock clocks with Realbench v2.56, then only proceed for 4.7GHz on all cores and so on.

For record, my 8700K has 1.088v VID and it's capable to do 5.0GHz running under a high-end custom loop at 1.280v on Prime95 v29.3 custom blend 1344K.

5.0GHz @ 1.344v seems like an average 8700K, so I think it's not going to run comfortably under air cooling/low end AIO etc.
What's your load temp in your daily applications/games and Cinebench R15?

So this is the result after 2.5 hours priming with the NH-D15S... maximum temp hit 91C

[attachmentid=9473597]

What's your RAM model specifically?
Activating XMP profile will automatically set your RAM speed/timings to the rating and voltage to 1.35v, VCCSA and VCCIO will be determined by your BIOS which might be excessive than your settings actually required.
You can try reducing or increasing them depending on the strength of your integrated memory controller (IMC) that's built inside your CPU IMC strength may vary from one chip to another so you will need to figure it out yourself by trial and error.
Obviously, weak IMC will require you to pump more VCCSA and/or VCCIO and vice versa.