spetz

I have an E220 coupe, and sometimes when the engine is hot and I start it, the rpm surge to about 1,500 and then drop really low (or at least that's what the tacho shows) and then immediately stabilize.

This whole thing happens quickly, so by the time the engine starts, revs go to 1,500 and drop down, and then stabilize to normal idle is about 1 second or so.

What could this be?
It only ever happens when it's hot.
When started cold the revs just go to normal idle and sit there. It never has trouble starting, cold or hot, a second or less of they key turn is all it takes to fire it up.

Thanks for any help