This is guessing. Could something be off in the accelerator linkage?
I was wondering if faulty timing in diesels can make them overheat as is possible with gas powered cars. But your overheating is intermittant. If the accelerator linkage is catching or slipping maybe the pump and the throttle aren't working together the way they're supposed to.
Also - what was the last work done on the car? Sometimes the root of a current problem is in the last work.
Good luck and hope whatever it is is minor,