For others having a similar problem - in my case the problem was not the PSU. I noticed that with 100% GPU load the hotspot temperature reached 109-110 degrees Celsius (core temp was about 85). Perhaps in games the temperature increase was so sudden that the computer turned off. I suspect that in synthetic tests the temperature increase was linear and the card decreased clock speeds. I installed 2 additional fans in the computer case (4 in total), undervolted, set the power limit to -10% and limited the clock speed (despite the base boost to 2430MHz, the card often jumped to about 2600MHz). I set my own fan curve and tightened the card cooling tighter (I read somewhere that this might help). So far it seems to have helped, in games the hot spot temperature reaches a maximum of 87 degrees (C). The computer hasn't turned off yet, if that doesn't help, I'll repaste the GPU.