1988 240DL B230F

I have an intermittent high-idle. The car starts, and runs, and then all of a sudden the engine will accellerate on its own and then come back down. When I accellerate the engine (while in park) and take my foot off the pedal, the rpm comes down to a point and then goes up for a couple of seconds before coming all the way down. Any ideas?

(Background: I've been having intermittent no-start problems, and for the time being the car is starting ok. I've changed the plugs and wires, distributor cap and rotor, tested the coil. Those all seem to be good, unless its possible they can be "good enough" for the car to start sometimes, but "bad enough" to not start all the time.)

Thanks.