I am going to guess you either have a 2011 MBP or the 2007/2008 MBP since they both had graphics issues.
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I am saying that changing the paste is a great way to lower temps, as well as taking care of your fans (replace when bearings start to go). Either a 17" or a 20" is a difficult repair to do just for paste, but it may be lucrative to do. Get some paste on that G3, you are asking for trouble with it.