OK its like this, it restarts when "extra" electric power is needed. (disc drive, hdd....)
2 solutions,
1) clean all coolers, remove power supply, open it clean it
2) theres a chance your power supply is dying, when u clean it check for swolen(enlarged) capatitors, solution get new power supply, aim for a little higher powe than yours currently.
And yes, if u got a fan on graphic card check is it spining, maybe its stuck with dust and when u enter a game(or app tha uses extra graphic) it overheats very fast, and crash.
cDa