Likely. The only way to make sure it's not overheating is to check the temps. Use RivaTuner and run it in the background and make sure that they don't get above 90C.
Other than that it could be a low power issue, but I'm betting temps are the culprit. The card will heat up really fast under 3D stress, especially if the fan is dead or the heat sink has dislodged itself somehow.