91' 300E starting mystery

[Gill]

After sitting for a few days my 91' 300E starts ok but after driving for approximately 10mi and coming to at stop at say a red light it starts to sputter and then stalls. It turns over but will not start after that but if I leave it for an hour or 2 it starts right up like there was nothing wrong. This only seems to happen if I leave the car sit for 3 or 4 days. If i drive it every day I never encounter this problem. i've had a mechanic look at it and he failed to find a problem . It has happened numerous times after the car has sat and just happened again on my way to work. I went back and got the car to drive it home . It started and it is idling and running fine now.Anyone have any idea what it could be as this is driving me nuts...

Thanx
Gill