I have a 1993 929. Occasionally, usually when I'm trying to start the car, it will idle up and then the idling will go way down and the car will stall. I'll start it back up a couple of times and rev the engine and then take off and go. It does this very rarely. When I'm sitting in the car when stopped, the idle will go smooth, then about every other minute it will do a quick funky thing where it acts like it's missing a beat, and the idle will go lower, just enough for a quick jump, then quickly regain it's normal idle. Never stalls when it does that, just does the funky bump thing that lowers the idle for a quick second. Idles very nicely majority of the time, but then that little bump.

Any ideas what that might be? Just had the timing belt changed, so doubt it's that. Plugs? Anything else?

 

That had happen to my 626 for a long while and I couldn't figure it out until I found out that air was leaking through my intake manifold gasket. The way to check is to get throttle body cleaner and spray around the area where the manifold is connected to the engine. If your idle jumps up all of a sudden out of know where when you spray, that means that air is leaking into the spot where you last sprayed. If this happens, spray the area again to see if happens a second time....then you definitely know there's a leak. Gaskets are only like $6-7......If this is not it, you might want to check the condition of the MAF sensor or even the throttle position sensor. Good luck...let me know how it goes.