Hi Dominic, :smileyhappy:
This is the correct forum, now worries.
I understand that there was a overheating issue and you have replaced the fan ,the heatsink and thoroughly cleaned the notebook, but you are still having an auto shutdown issue. At the customers request you have replaced the hard drive also.
Before replacing the motherboard, I would run the diagnostics to test for hardware failures. Testing for Hardware Failures (Windows 7, Vista).
I hope this helps.