I can't answer your question directly, but I'm experiencing a similar problem with idle surge on our '93 300TE wagon (W124 chassis, 3.2L six). Do a search here for OVP, also for idle speed, idle surge or surging idle. There seem to be two culprits - the OVP relay and the idle speed control switch (valve? module?)
1995 E320 Wagon Moonstone Grey/Parchment 106K mi.
2007 E63 AMG Graphite/designo 75K mi