My 85 740 turbo when starting the rpms go to 1500, then drop to almost 0 then they will even out at 1000. When driving coming to a stop they will drop to almost 0 then bounce back to 1000. Normally this car would idle at 800. This problem surfaced after the car quite at the side of the road. It would run but barely, but the next day it ran well enough so we didn't have to tow it. Had a problem with car dying in hot weather, just replaced in tank pump, mas air sensor connector and high tension lead from coil to distributer.

 

Replaced idle control valve and problem solved. I also replaced some poor vacuum hoses just for good measure.