Issue with +3.3V power supply on motherboard?

I have had an ASUS P5PE-VM motherboard for a few months now and I have started to experience computer lockups whenever I do something "resource intensive" - playing games mainly. I have now reliased that the +3.3V power supply is dipping all the way down to about 2.8V just before the lockups occur and I'm fairly sure that it's related. I have a Thermaltake 450W PSU so I'm positive it's got nothing to do with that. I just don't know what is causing it or how to fix it. Can anyone help?