TriggerFinger

So it is safe to assume you have run the clean up of temp files, and also have physically cleaned the vents/heatsink. Btw, the temps you posted previously are still on the high side. But since you said you have followed the instructions on the link, this could mean you may need to reapply the thermal compound or clean the laptop from the inside.

Run Memtest86+ to test your RAM and also run HDD diagnostic utility to test your drive... if you have not done so yet. If you did, run CHKDSK /R using an XP install CD (if you are using XP).

If you pass all the tests above, connect your laptop WIRED (DO NOT USE WIRELESS) to the internet and use a linux live CD (try PCLinuxOS). Since this is a live CD you do not have to install anything on your hard drive. Boot with the PCLinuxOS live CD and go to YouTube and see if it slows down after 5mins as you have previously experienced.

Post back what you find out.