Check for heat problems as well....clean out the insides with compressed air using a QTip to hold the fan blades in place when blowing them out...
And while the side panel is off by all means swap in a known good PSU of the same or greater wattage to see if that's the problem...
The other possible culprit could be bad RAM.
Just an afterthought...the card could be going South...we'll cross our collective fingers and hope this is NOT the issue.