The iMac was crashing intermittently even after OS reloads, sat for 4 months unused until I had to replace the power supply on another iMac.
I forgot to plug-in the temperature sensor on the new power supply resulting in the fan (near the bottom of the imac) running at full speed continuously. Take apart and reassembly fixed the issue once the sensor was plugged back in.
iMac is now up and running with OS 10.8 for over a day without any issues.
The old power supply had a leaking cap that would appear to be the issue with the whole computer.
Lay out all the screws on a piece of paper and mark which step they came from, the power cable to the LCD can be very tricky to put back in place.