This sounds like a loose connection problem. Check all your connections carefully and continue running the computer open for a while (until you feel confident that the problem has "worked itself...

From reading through a different thread, I learned that sometimes the heat transfer cream between the CPU and the heat-sink dries out. When this happens, even if the fan is working, the heat will...

You seem to have done the right things so far. Now all that's left is to replace the fan once to receive the replaement one you ordered.
I have a thought about something you might try until you...