1987 300 TD, with 88K miles, has started to exhibit an elevated idle speed (approx. 1200 rpm), in neutral and in drive, at the end of non-stop 200 mile interstate drives. When allowed to cool overnight, the idle speed returns to normal and does not recur during city driving. With the exception of the elevated idle speed, the engine behaves flawlessly, with no other symptoms present.

Any ideas as to the cause of the idle speed elevation?