The GPU overheats and the laptop shuts down

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http://superuser.com – I had to replace my laptop's graphics card a few weeks ago. I also had it cleaned. At first it was fine, except the fan was loud. But since yesterday it has shut down twice. Once when I tried to play the sims 3 and once when I was watching an hour-long video on youtube. I've been monitoring the GPU temp for a few hours. It's always at ~70 degrees and if I start the sims, it goes above 90 right away. I can't take it to be repaired until the end of June because I'm in the middle of finals and I can't do anything without my laptop. I'm hoping that it's just the thermal paste but I'd like to know (HowTos)