Yes heat.

Do you have a NVIDIA videocard? If so, Forceware drivers have a simple temp monitor built in. You can also obtain http://www.guru3d.com/article/nvtemplogger/294/2/

My motherboard had its own sensor tool, so I'm not sure what to use.
http://downloads.guru3d.com/download.php?id=8 lists some general tool. You want one to monitor your CPU, mobo and maybe even Northbridge. http://downloads.guru3d.com/download.php?det=893 is what you need I think.

You can use Asusprobe (supplied with driverCD) if you have an Asus mobo.

I'ld say it's your gfx cooler. I have a Nvidia 6600GT and it hit 90+ degrees Celsius when running games. The stockcooler is poor and it performed even less after a year. So games started to crash. Installed a ~$20 Zalman and it's 60 degrees Celsius now.

Also try to keep your case clean from wires: bundle them with fasteners, put unused cables away, move some devices, add a casefan.

P.S. if it is the gfx card, just opening the case and blowing with a fan might not be enough. Those chips can get very hot and a poorly installed heatsink/fan may block it from any external cooling.