Why does my notebook keep overheating

I'm not sure if anyone else with this laptop has this issue, but It didn't start till about a year ago. When I am just idling it will be ok, and if I run a heavy benchmark and push it to the max its also fine. But when I am doing something that's not pushing it to the max like gaming, or virtualizations, or even just windows upgrades it will raise the fans slightly and then crash hard. like to the point the entire system is frozen and will not respond (tried.. it wouldn't respond for an entire week) unless I hold the power button and force it to shut down. at first I thought it was because I upgraded from windows 8.1 that was originally on this... but after downgrading back and installing all original drivers it was still doing it... So then I thought it was the heat so I went and pulled the back plate off and saw that what I believe was the Nvidia 650M GPU had a heatplate "above" yes above it, no paste and the plate wasn't even touching it. it was attached to the CPU heatplate and stretched down to that but wasn't touching, just hovering.. so I put the correct amount of heatsink paste on it and made the contact. Thought that might do the trick... sure enough.. it didn't... it still does it.. I'm like ready to throw this thing out the window... like I paid 2000 bucks for this and it starts doing this just out of the replacement year.