I recently wrote some Java code to generate images of the Mandelbrot set (fractal). I made use of the new Fork/Join facility in Java 7 to run separate threads on all four cores (2 real, 2 virtual)simultaneously, using a large number of iterations for greater accuracy. The problem is, the process runs fine for about a minute, and then it's as if someone has pulled the plug and the PC just dies.

I thought it must be the CPUs overheating, so I ran Real Temp to monitor the temperature. It's an Intel i3 processor. I can see the temperature creeping up to 70 degrees, and then it seems to level off there and run for about another 30 seconds before dying. According to Real Temp, there's still a gap of 35 degrees between the actual temperature and TJ max. I also tried disabling "CPU TM function" in the BIOS, but the problem still occurs.

A colleague suggested that it might be a power supply problem, so I borrowed a more powerful PSU (can't remember what wattage it was, but it's higher than mine which is 500W). The exact same thing still happens though.

Is anyone able to suggest what the problem might be, or what I can try next?