|You could try running a memory test. Or you could try snapping the RAM modules in and out 4 or 5 times to burnish the contacts. Your rig is over 5 years old. The contacts may be oxidized. One memory tester is at the link below.|
Also visually inspect the motherboard for any bulging or leaking capacitors. You could also re-seat all connectors and add in cards. One last thing you could check is to see if the voltages are up to spec in the BIOS, if they are visible there. Voltage readings should be within +- 5% of the rated output.