When was the throttle body last cleaned? Idle is maintained by controlling air flow on the intake. With the a/c on, more air is introduced by the idle air control motor. With a/c off, more of the idle control is dependant on the throttle body, which builds up deposits over time at the butterfly valve. These deposits ruin the idle on an otherwise good-running engine. A good cleaning with solvent and a toothbrush can perk up the idle.
95 E320 Cabriolet, 131K