83 500SL Idle Speed Puzzler

[CaptnAl]

A long-standing problem with my Euro 500SL is the idle speed, which starts out fairly fast (1000 rpm) when cold, steps down to an appropriate speed after warmup, but about 20 minutes into a drive, the idle speed drops too low (about 450 rpm). The car can stall at a stop sign, even if I've shifted into neutral. The puzzling aspect comes from what I've replaced, with no change in these symptoms. New idle control valve, new electronic idle control unit, and confirmed the contact with coolant temp sender at the thermostat. The idle speed does respond to gear shift position and A/C being on.

Thoughts or experiences that might help??