It's probably the ram I think one the ram sticks may be bad it was giving me trouble booting when I added my CPU I had to take out my ram and reinsert them. REALLY old DDR3 sticks 2gbs
I have Dell Optiplex 790 MT with an RX570 8vram, 8gbs ram and a recently installed stock i7-2600k and I'm getting unstable fps in most games especially in Destiny 2 . I've checked CPU temps and done a few stress tests, there isn't any thermal throttling, turbo boost and hyper threading is on...