Just a few random suggestions in no particular order....
* In case of a bad contact, remove and re-seat as much as you can on the motherboard (DIMM, VGA, PCI cards etc.).
* If you have more than one memory module installed, try removing some and see if that makes a difference.
* Check that the CPU heatsink is getting warm and is making a good thermal contact with the CPU. If in doubt, buy a small tube of thermal grease from your local electronics store and clean/re-apply sparingly. Either way, give the heatsink a few gentle twists to bed it down.
* If the BIOS has a preset "Fail Safe" setting reset to those and see how it goes.
* Hooking up a spare PSU (even if it won't fit your case) might tell you that your PSU is marginal or failing.