Inspiron 17r 7720 SE 650m GPU overheating

I recently had a problem with my Inspiron with the CPU throttling when gaming. It resulted in skipping and lagging in games.

I took my laptop apart, cleaned the heatsink of dust, cleaned the old thermal paste on the CPU, GPU and Heatsink (there was way too much in my opinion) and reseated everything carefully, screwing down each screw bit by bit to ensure good contact.

After making sure the heatsink was fitted correctly and firmly, I put everything back together to test. The CPU overheating problem was fixed, my i5 3230m reaches only 70c load.

The 650m GPU however is a different matter, this actually is reaching dangerously high temperatures, during unigine heaven it reaches 92'c and the laptop shuts off, presumably protecting itself against damage.

I don't regard myself as an idiot when it comes to computers, I have built many before so I know how to correctly apply thermal paste, make sure heatsink is making screwed firmly etc but what could the problem be?

Do I have a faulty heatsink? The fan itself is working fine and hot air is being exhausted, it raises to max speed under high load.

What can I do to troubleshoot the issue? is it worth getting hold of some better, newer thermal paste and applying that instead? Should I order a new heatsink? Should I try using copper shims? A more powerful fan?

I wondered if anyone else who has had something similar in the past could help.

Thanks for the time and for having me on the forum!

~Dodge