1. Download and install RivaTuner: http://www.guru3d.com/index.php?page=rivatuner
2. Click this button and choose the last option. It opens up a clockspeed/temperature chart.
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3. From what I noticed, if temperatures go above 105, the FPS will have a sudden drop, and the graphics card might shut down completely if it goes over 115C. My idle temperature was around 78C and ramped up to ~110 while playing sc2.
4. If overheating is the problem, try to dust it out first. If your graphics card has an enclosed case like mine, you might have to pick out clogs of dust with a tooth pick. After all the dust was cleaned from my card, the temp went down to 42 idle and ~70 while playing.
5. If it is still overheating, new thermal compound might need to be applied to the GPU. Refer to various guide on how to do that.
6. If that's still not enough, try an extra fan for your graphics card (if it's expensive and deems one). People say good things about this brand. http://www.arctic-cooling.com/catalog/index.php
An alternative temporary solution is to underclock your GPU, which RivaTuner can do. Click the little button under driver settings and choose system tweaks for overclock settings. First try set all the numbers to ~2/3 of what they were originally, to see if FPS drops go away. This worked for me before I took out all the dust. Although temperatures were still high, it no longer dropped to 10fps. + Show Spoiler +