First I would make sure there are no air bubbles entering the injection pump. You can do this by removing the return line from the injection pump.Submerge it in a container with a little fluid in it and look for bubbles.

My second guess is that it may be the govenor in the injection pump. A good soaking of the pump on the car might or might not help.

I assume you have fairly normal fuel pressure in the base of the injection pump. Unstable idles have been reported with very low fuel pressure but I can remember no cases of hunting from low fuel pressure. Usually the idle is just a little rougher and noisy.