you obviously know your way around a computer and it seems you have tried the normal route of trial and error.
If i may suggest a couple of ideas for you to try;
Have you removed the cpu at all ? if so, did you clean the old thermal paste off and replace with new ?
This could be a case of overheating too.
Is the fan working ?
Of course laptops can be extremely difficult to actually enter to clean the fan, so a blast with compressed air may help out a bit, but this will not clear any dust residue just place it somewhere else on the motherboard but out of the fan.
Also try running Process Explorer from click here to check for any processes that maybe using your resources.
You can also reduce your startup programs in MSCONFIG to the basics, ie; antivirus and firewall and leave any real time scanner ticked.
You may have a buggy driver that is causing the problem, just another stab in the dark.
Obviously its a matter of elimination.
Does the laptop fly along ok in safemode ?
If its ok in safemode, chances are its a driver issue or one of your apps.