OVP relay dropping out?
I don't understand the intermittant vac drop with no idle speed change, either, this is odd. A vac leak big enough to drop the idle speed would make the engine go lean and at least shake.
Could there be an ignition fault here, so the idle control valve drops all the way open intermittantly? Hard to imagine that unless it also caused idle instability, too!
Could a bad O2 sensor do this by messing with the mixture?
Strange. Sounds like a huge pain in the sitting part, too!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!