Hi there!
When you did cleaned it, did you also replaced the thermal paste and thermal pads? If not, please replace it. The stock thermal paste is not that great, so replacing it should fix it big time.
Guide for replacing the thermal paste: [guide|139167]
You could also replace it for liquid metal, but this is more difficult and more work need to be done (isolate around the APU). I recommend to replace the thermal paste first and see how that goes.