This shows a fairly high end machine that may require some TLC every few months.
Did you try cleaning it like we see at http://reviews.cnet.com/4520-11319_7-6240575-1.html
Also, while you're inside, grab that CPU heatsink and test if it's solidly mounted. Not a lot for force needed but if it's hanging by one bolt or wobbling then you know what to do next.
Before the cover goes on, boot up and all the fans must be checked.