Honestly it doesn't sound like you really know what you're doing in there, laptop cooling isn't desktop cooling, doesn't really sound like the guy you took it too really knew what he was doing with it either. You need proper paste on both the CPU and GPU and thermal pads on whatever other components had thermal pads on them and they all need to be properly seated or you get the problems you seem to be getting. Bad thermal pads shouldn't cause 100C CPU temps (I assume you meant 100C earlier), but they could still cause instability and crashes, and just because screws are tight doesn't mean they're properly aligned, if something is slightly off you might have an air gap and be tightening down on a wedge of some sort.
You know what? Because i knew i know nothing in laptop cooling, i went to some supposedly reputed computer repair shop to have them solve the issue, but they screew the job. They changed thermal paste and assume that my overheating was solved.. Went to another shop and say that he "think" that the heatsink might be the problem (after dismounting and checking mostly everything).... Thats why i decided to come here. I surely dont pretend to know much about laptop cooling
(Yes 100C)