I was about to give up with a Sphire Nitro+ 5700 XT. Random reboots when playing, using windows etc. Tried all possible drivers, tricks, windows hacks. Nothing.
Then, I found a post about undervolting the card, I was suspecting a PSU problem, but having a 750 PSU when the recomended setting is 600W, I didn't want to believe. I tested the undervolting and the card started to work better, to the point I was able to play with not crashes. I ended up replacing my 750W bronze PSU for a 850W gold rated PSU. Clean windows install, latest drivers, everything worked as it is supposed to.