I was going to suggest that it's either the PSU or the RAM, and to try running a memtest86+ session for an hour or so. But try resetting the BIOS to its defaults (or "fail safe" on some versions).
Unplug any peripheral you don't need, like webcams, microphones, external drives, USB hubs, etc. They could be draining too much power from the PSU.
It could also be the motherboard itself. Inspect it for any fat or rounded top capacitors (if you know what a capacitor looks like). They have a little cross at the top and the top should be flat, not blown out.