C200 stalling problem

Hi all, wondering if anyone can help me with a problem.
I have a 1996 C200 Esprit (auto petrol) recently I have been experiencing instances of the engine stalling. It happens when I slow down for traffic lights,junctions etc, anytime I have to come to a stop while driving. It doesn't stall when I actually stop but when I go to pull away again, also the rev counter is behaving oddly. When I stop (whether I go into neutral or stay in drive) the engine seems to rev on it's own, up to about 2000. It comes straight down again but is pretty worrying! I have taken the car to my usual mechanic who hooked it up to diagnostics which showed no faults at all. He also drove it and didn't experience any stalling. It happens whether the car has just been started up or has been driven some distance. I'm so worried that I'm going to get driven into if it keeps stalling, I'm actually becoming a little scared to drive it!
Other than this problem it's a lovely car with no loss of power and no other problems.
Any help or suggestions would be most welcome, thanks in advance!