Dust!!!!Get a screw driver, flip your baby over, and unscrew the fan from the laptop its on the spot opposite of where it says Inspirion | 8600 on the top left of your keyboard. When you do there will be a crapload of dust blocking the exit of the heatsink grille.
I know, I had the same problem and I would get massive fps problems after 5-10 minutes of 3d gaming (it would skip sporadically). I recently took apart my inspiron to clean out gunk and lo and behold, the fan was COMPLETELY blocked by dust gunked up for both my cpu fan and my video card fan. Now everything runs much cooler.
BTW, I game on average 8 hours a day and had my Inspiron for 4-5 months when it started happening.
After you clean out the cpu fan, you might want to take the keyboard off too (ala the instructions in the mod guide) and clean the dust out of your video card fan for better gaming).
If dust isn't your problem then I dunno though.