Also, if the CPU is heavily loaded while, at the same time, the GPU underutilized - that is a clear sign of a CPU bottleneck.

If the stutters occur at the beginning of the game, the probability of the culprit being high GPU temperatures is decreased since the GPU had no time to get fully heated from operating under heavy load.

And if the card has a fault running something like Furmark might kill it, so do it at your own risk.

Hope that helps.