After reading all the replies I got (thank you support, very helpful) and much deliberation (and by that I mean relentlessly fiddling with Crimson) I think I found out the problem. Every summer, since its hot as hell's deepest pit, I open the PC's case. I did that before swapping the GPUs. But after changing the temperature control with Wattman, the crashes stopped altogether... Even with the case opened the damn thing was overheating, probably because of Wattman's poor control of fan speed, and since I keep the case open and the fans were not noisy I didn't even consider it could be the temperature... Now the whole thing here is noisy as a pig at the slaughter house, but at least it's all working now, and no need for OS reinstall.