I do not know anything about the Autronic system and its features. I am running a different, after-market, fully programmable system. The mapping of the idle region was one of the trickiest. In my case I had to run the fuel mixture a bit on the rich side. Also, it was recommended to spread the 4 adjacent map points out far enough beyond the target idle rpm and manifold pressure to keep the idle from hunting around. These table points should also have the same values (a kind of flat low spot in the table curve). For example, if the target idle is around 900 rpm then the the idle points on either side should not be 890 and 910, but further apart, say 700 to 1100. The goal is, that if the idle drops outside this flat region than the adjacent settings will increase or decrease fuel causing the idle to return to the desired region. If there is too small a region then it can pass right through to the other side - and go back and forth (hunting).
Another possible issue could be the engine rebuild. Can you find out what was done on the rebuild? For example, if a higher degree cam shaft was used then the idle is more difficult to dial in, as the engine does not perform as well at low rpms (so I have been told). I do not know what other engine mods would have similar effects (too light a flywheel?).
It could also be that the cold starting valves or EFI settings needs some work. The MegaSquirt system that I am using has various tables for the cold start condition that adjust timing and fuel mixtures based on temperature and loads.
If the rest of the car is sound, from a rust, paint, trim, and interiors perspective, then it seems that the fuel injection system is a less critical factor. You could have a different system installed, especially since most of the hardware elements are already in place (pumps, injectors, etc) - the Motronic 1.3 is popular and works well on these cars.
Good luck.
