Check to make sure the insides of your computer are clean and the heat sinks are installed properly and with the right heat sink grease. Depending on just what your hardware is, there may be software that can monitor the temperatures and let you know what is going on.
My computer restarts when playing games such as Combat Arms.
It occur after about 10mins of playing.
Whats ironic is,i recently had my computer re-format.
And my com have lots of free space.