My solution to the exact same problem
Cleaned the Idle Control Valve with carburetor or brake cleaner. Then lubricate it with WD-40.
I had to clean it in 2 consecutive weekends, the problem has not happened again.
Do a search for Idle Control Valve, and you will find more info on how to clean it.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car