Gateway randomly shutting off (overheating?)

Hi everyone.  I have a Gateway T-1625 that has started shutting off randomly.  I noticed that when it shuts off the bottom of the laptop is really hot, hotter than normal.  I'm guessing it's overheating.  I've run some different utilities to see what the temps are like while doing stuff and it will go anywhere between 65C and 90+C.  I took off some of the stuff from the bottom of the laptop to make sure the fan was working and it is so I'm not sure why it's suddenly began overheating and shutting off.  Any ideas on what to do?  On another forum I saw someone suggest to another person having an overheating problem to put some arctic silver on the processor. I actually do have some and could possibly do that if you all think that might help.  Somebody else suggested updating the BIOS because maybe it will fix a fan issue or a processor issue with the processor not throttling back to cool off a bit.  Let me know your thoughts.

PS - While on this form the temps been hanging out in the mid to upper 80's.  I'm not doing anything on the laptop but this right here. 

Interestingly enough the laptop is running perfectly fine now after I took it apart yesterday afternoon.  Very strange.  At first I thought it was the AC adapter causing the temp to rise so much.  I thought this because when the temp was hovering in the 80's I disconnected the AC adapter and the temp immediately began to fall, and once it was finished falling it was hovering around 60C.  However, an hour later when I reconnected the AC adapter to see what would happen the temp remained around 60C, so I guess it might not be the AC adapter after all.  What a weird situation.  I'll keep an eye on the temp for the next week or so and decide where to go from there.

All I did was open it up.  I didn't apply any new thermal paste.  Strange...

Hi all.  So it's been a month and a few days ago this all started up again.  I've noticed that every time the temp starts going up it will go down immediately if I unplug the AC adapter.  Any thoughts on what might be wrong?