About 1.5 years ago I purchased this GTX 770 DC II (GTX770-DC2OC-2GD5) and noticed it ran on the hotter side, low-mid 80s C at 100% load. Until recently I hadn't worried about it because none of the games I had been playing were very demanding, but when I noticed it running hot again a couple weeks ago I decided to see if a TIM change would help. The stock TIM was a little messy/too much, as expected, but not terrible. While I had the heatsink off I also polished it starting at 400 grit and worked up through several grit to 3000 and a mirror finish. For the first few days after doing this I noticed the idle temp dropped from around 38 C to 32 C and at load it dropped from around 82 C to 60 C. In fact I let FurMark run for about 15 minutes and it stayed at a steady 60 C, but just 4-5 days later I noticed poor performance in a game and saw that the temperature was up to 93 C just seconds before the screen went black and the computer froze. I redid the thermal paste a second time and had the same results: low temps for a few days followed by a spike. The TIM I've been using is Arctic Silver 5. Since the heatsink is polished I only applied a very small amount to the die. When I redid it a second time the paste seemed to be thin but evenly spread across the entire die surface. I also noticed that after these instances of high temps the heatsink itself seems to be slightly loose and feels like it glides on the thermal paste despite being fully tightened down. The amount of metal taken off of the heatsink couldn't have been more than the thickness of a piece of paper so I don't see how that would cause it to be loose. The only think I can think of is that the stock TIM was more like an adhesive thermal pad so that despite being thick and making a mess it helped to hold the heatsink in place rather than drifting around like it does now.
I appologise for the very long post, but I thought I should include everything I could think of. I'm completely stumped on this one so any insight would be greatly appreciated.