You can hold the SHIFT key down to bypass all extensions to see if you have an extension issue or some hardware issue.

Have you bothered to open the case up to see if anything looks funny internally? Have you changed anything to make the system unstable (software or hardware)? 840AV motherboards have been known to die from capacitor leakage, see if you have any major areas where dust seems wet around capacitors. Could be power supply related as well.