I have bought a Dell XPS 15 (l501x) laptop 1.5 years ago,
some months ago I have experiencing shutdowns and I cleaned up my laptop fan and heatsink and my problem goes away.
but now i have same problem, my laptop shutdowns very quickly, even on diagnostics! my cpu temperature quickly goes to 100 centigerad and shuts down.
my gpu temperature is fine (about 60 degree), my fan seems working properly. i replaced cpu and gpu thermal pastes but is see no difference. i tested another heatsink on my laptop too but my problem still remains.
something that is strange for me is difference of temperature between top of cpu and 5-10cm of near it.
temperature of my heatsink on top of cpu goes to 90 degree but near of it in 5-10cm distance is about 50 degree.
I also had issues like that, replaced HSF (fan had died) and couldn't get temps under control even with new HSF. Finally (after three disassemblies) I replaced the thermal pads. So, I have to ask, have you replaced the thermal pads or are you reusing the ones that were already on it (I believe a total of five). I read in the past that if one is careful, the thermal pads may be reused - I tried that and found I have no such luck.
I'm going to suggest you try that route if you haven't already.