I can't, too late.I'm not sure what else you can do.
Like I said earlier, when you used another card and the problem was resolved that's the only true answer to your problem.
There is something going on with your video card that is causing the issue. What it is exactly is still a mystery.
Drivers don't fix it.
Any software tweaks don't fix it.
Using another video cards fixes it.
Can you take the card back?
But, if to be more accurate, AMD GPU almost (key word) haven't frame time spikes. They were less noticeable and frequent in GTA 5 though, but I couldn't test AMD in The Witcher 3 or other heavy games because of the card weakness. I've only tested AMD HD7770 in:
- GTA 5 (almost no spikes)
- CS:GO (no spikes as it's on 1080 Ti)
- TF2 (no spikes as it's on 1080 Ti)
- WOT (no spikes as it's on 1080 Ti)
Maybe it's game related, because GTA's engine is full of hitches (with 188 FPS cap) due to poor porting from XBox. Other games (TF2, CS & WOT) work well on GTX also, not on AMD only. IDK what the hell are that spikes...