Hit the above, select Performance and see how hard the CPU is working, also how much spare memory you have. If this is clogged up, you have something working in the background eating resources.
While your at it, check Applications and Processes to see if you can see anything there that you do not recognise.
If this all appears clean, you could try running a memory checker, to see if that's gone "duff". If you were on the right lines to start with, (a thermal problem), stripping and cleaning the unit could work miracles, but has to be done with a great deal more care than perhaps would be necessary with a desktop. Plastic easily breaks!