Investing in a dash board read out for codes ; SilverLeaf , Scangauge D ; so you can get codes as they happen , before you spend any more $$$ on un-necessary parts replacement.
Coolant level sensor would have been my first choice for a sensor replacement , but there should have been a second dash light ; Low Coolant : and " CEL "as well as the de-rate.
From my experience with Cummins , ECMs and VP-44s , they are seldom intermittent failures , once they go bad