check the idle control valve, this is very accessible near the front of the engine to the right of the thermostat. New valve is about $240 online, same one as MB, it is made by VDO. Maybe FastLane on this site has it. MB charges about $700 for the same part.

Make sure all vacuum hoses are connected and intact.

I am not sure if the warmup regulator would contribute to what you describe.

After that test/verify idle computer followed by ECU.