I use Arctic,
https://www.arctic.ac/eu_en/products/cooling/thermal-compound.html. For whatever reason Byrd appears to have a second opinion on my preference. In any event … any brand of thermal paste will beat whatever is still in your machine. Whatever you consider, maybe read some reviews to get a second opinion.
@Byrd brings up an excellent point, one thing I overlooked are any 3rd party cards. Here's what you can do … remove any 3rd party card or other internal device and for that matter apply the same consideration to external devices. In other words try to reduce the hardware configuration to as close to an out of box situation as possible. After you've done this, try to reproduce the issue. Give it as much time as it would normally need to reach that same heat level that you initially reported.
If the issue can not be reproduced with those internal/external devices removed then shutdown, add one device at a time and again, try to reproduce the issue. If you eventually find that a particular device does provoke the issue to be reproducible consider any outdated software that corresponds to that device.
As Byrd said, something may keep waking the machine while it sleeps but those resets, if you've done them correctly are there to correct sleep issues. Well not all those links refer to that issue but one of them does.
So, even though you should apply new thermal paste, you should also put an effort into understanding what might be triggering the machine to potentially be waking from sleep.
If you need to test with clean software, when it is a point of suspicion, you can always boot from a clean install on another volume or from optical.