Um, your computer's too hot?!?
1) Open and CLEAN IT OUT INSIDE (canned air @$5US is a bargin for this!) Be sure gratings/openning are clear of dust/lint.
2) Be sure FANS ARE OPERATING (Esp. GPU fan on G-card iff any!)
3) Check that nothing has fallen down behind/around machine and blocked airflow near/around case.
Extended use of the card when overheated will, of course, ruin it at some point (if it hasn't already....
)