If you've already replaced the power supply the check the obvious....cpu cooling, memory, MB battery. Depending on how much ram you have, pull all but one stick and try it . CPU cooling is the most important item to check. In the time it takes your pc to post on a restart, it can burn up if your heat sink / fan are not working. You will never be able to get into the bios to check the temp of your cpu if it is overheating. The thermal sensors built into today's processors automatically shut down the pc if the cpu overheats. That is so you don't burn up the cpu. If all is ok with the cpu, then check the ram, one stick at a time. Let me know how it goes, and we'll take it from there.