I just tried loading Puppy linux from a USB. Didn't work on this machine - but does on others. So something must be wrong with the PC. Could it be the motherboard?
It could be a lot of things, but I'd be inclined towards running memtest86 on the RAM cards, first as a group if there's more than one, then on each individually if any errors show up. I'd also check all of the connections and sockets, by removing each add in card and reseating it, and each cable connection, at both ends.
I regularly tear my systems down completely, do a thorough cleaning of everything, then put everything back together again. I run memtest86 to be sure the cards are properly seated and in good condition. Most cables get replaced with new ones if there's the slightest doubt about their integrity. It's a bit of a pain, but I also have far fewer hardware related problems than anyone else I know, so it seems to be working.