Like many people I waited too long to replace my GTX 980 Ti. I've been getting hard reboots and some stuttering after an hour or so gaming. Temperatures are ~82 C on the GPU and ~70 C on the CPU. CPU-Z says the GPU fan speed is only 60% but its being thermally throttled.
 
That's too hot for the GPU right? I figured I'd get artifacts or other crashes, not hard reboots, but... The PSU is from 2014 but is 1000W so I assume it still has some head room. The only other thing could be the motherboard and CPU, bought together about 10 years ago (i7-2600k). This year feels like a bad one to buy a new computer that'll last.
 
I've done the usual air dusting everything out and already replaced the paste on the CPU. I was thinking of replacing the thermal paste and pads on the GPU but I'm not clear on the pad sizes. A recent EVGA Q&A says they're always 1mm but a forum post from the era said 0.5mm. And I'm not sure what amount to get. Does anyone know? GTX 980 Ti 06G-P4-4998-KR 6GB CLASSIFIED.
 
I'd appreciate any tips on reseating the heat sink too. Thanks!

The case is a Coolermaster Cosmos S. The 200mm fan on the side sucks air in and two 80mm fans on the top push it out. The CPU cooler is the low profile stock one (I always meant to get a larger heatsink but the CPU is near the end of its life now). Ambient temperatures are around 75 F and there's plenty of space around the case. I do need to get a few new 80mm fans as these are starting to wear out and I could be using the rear and bottom mounts, so that's a good reminder. I've assumed since I only run one GPU and don't overclock my CPU anymore that I didn't need the airflow.
 
The GPU temps are what I was trying to understand more about. I meant to say I was using GPU-Z, and it says it's thermally throttled at 83 C. That is pretty hot for a GPU right? I did play with EVGA Precision to throttle at 79 C and that seems to help stability in the one short session I've been able to play so far.
 
Regarding replacing the TIM, I thought if I popped off the GPU cooler that I would have to replace the pads over the RAM as well and I wasn't sure what to get; is that not the case?

Thank you. Sorry for the super late response. Yes you're right they're 120mm fan on top, not 80mm, I wasn't paying attention. I've been running them higher and it's helped; I'll replace some of the missing ones soon. Thank you for the link showing the cooler removed too!