Frustrating type of thing. I know nothing about the injection pump on the 603. Other than I thought there was an electrical idle control. What happens when this is disconnected?

A wire going to the injection pump I believe. One could monitor the voltage there and substitute a fixed one if required. That was if that voltage was fluxuating rapidly when warm but not cold.

I also wonder if the rack damper bolt needs adjusted if they have one. Mark your starting position so you can get back to where it was if of no help. Just a couple of things that perhaps the average guy can check out.

You also have more experience than the average guy. I doubt there is anyway to detect this type of fault with an injection pump on a test stand. I might try disabling one injector at a time to see if the problem stops. I doubt it will but once again for the time spent.

Basically just some easy quick managable shots in the dark. That probably will show nothing. Brians thoughs on air ingestion has a better possible chance. With air I would expect more of a constant rough idle though. Check your overflow for fuel expulsion as well. If for some reason the pump was being partially starved of fuel it might be erratic I suppose.

Also I do not know if it could create this type of problem. Still would want to make sure the egr valve is not acting funny. It normally does not activate when first starting the engine. I think there is a thermal control. This is a really an outside chance type of thing.