So the heat sink compound is still years old?
And I'm unsure if you reached in there to see if it's on solid. I've lost count of how many have one of more of the retaining pins that popped loose.
Let's say it is the OS. A fresh install should clear it up unless it's the usual age or other issues. This is easy to do today and cheap compared to a repair counter. I use another hard drive in the shop for this test.