Update:
I went down to the service centre and told them about my prob.
The engineer reseted pram and advised to update my fcp and said it could be a software problem or kenal problem. He was very sure that it couldn't be the thermal grease cos he said he applied it very carefully. Also he opened up all my apps and stressed the cpu but the temp was at 70-80 degrees celcius only. Then he said it could be fcp prob and told me not to worry cos the cpu can actually go up to 140 degrees celcius!
So i went back and updated fcp but still same prob.
i dunno if i should send it down to the same service centre cos the engineer seems to be unsure of not what is the problem also and is hesitant to order another logic board to replace.
Any advice on what to do next?