If this is a djet, what you describe is vey common.

More than likely the AAV (aux air valve) is stuck and not opening or closing smoothly. You can remove and test in hot water or with a hot air gun. It should be fully open at room temperature, then as coolant heats up, it should be fully closed at 150F. That surging is often a symptom of a stuck AAV. You can usually stop surging by reducing idle rpms, but then you may have more of a problem when hot. Happens a lot. AAVs are expensive if you can find one. Try to fix the one you have.

You should check your fuel pressure - put any simple pressure gauge at the end of the line to the startup up valve. Turn key on - you should see about 29 psig. If you set it using the fuel pressure regulator, about 32psig might help with the hot starts. Car will run a little rich, but more likely to start.

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5