Specifc game shows very poor performance after installing another game

I have a GTX 550Ti that has 2GB of video memory.

One of my commonly played games is Minecraft, and the average frame rate on highest settings was around 170.
I later installed Portal 2, played it for some time, and following that, Minecraft no longer ran higher than 40FPS.

The max temperature hit while playing Portal 2 was 82C. This was due to inadequate cooling, but the 550Ti's max temperature is 100C.

I first assumed that that 550Ti had degraded, but other games such as TF2, as well as graphics benchmarking show no difference in performance than when the card was first purchased.

What is a plausible cause to this? Software settings have been reset, and I've also tried reseating the card, as well as different PSU connectors just in case one wasn't working correctly. The result is still the same.