I had something like this with my PC not too long ago.
For me it was the motherboard
It'll work maybe once in 10+ times, until it totally stops booting
(I tried ur RAM swap too and same results, suddenly works then not anymore)
For my case I have a blank screen, after a while fan suddenly goes to max speed and stays there until i force shutdown
Led and fan on and off repeatedly..no post, Ty for your insight..