Just curious as to what you mean by “tried thermal paste”. Could you describe the steps you followed and what it looked like (was there extra paste squished out from underneath the heat sink?) Is there a heat sink on it?
When was the last time that it worked perfectly? Did it get jarred or something spilled on it, maybe a power surge?
When you look at the main board closely do you see anything that looks like it doesn’t belong? Blown capacitors, black or scorched areas, areas with dust blockages, etc.
Does it power down instantly or does it initiate a proper power-down sequence. Are there any noises right before it turns off? (bzzzz, whirrr, bonk, tink, pop) I know it seems unprofessional to describe things like that but it actually can help alot.