99 S500 engine stalls frequently
 

My '99 S500 has started to stall very frequently at low RPMs. It always starts right back up, but it is quite inconvenient! Ive been watching the temp gauge and the problem does not appear to be related to engine temp. It usually stalls when idling at a stoplight, but sometimes when driving at low speeds the RPM just drops and the engine stalls.

The car has about 150K miles on it and recently had a complete valve job and all the vaccum lines were replaced, so I don't think it is a vaccum leak.

There are not any engine codes to give me any clues to the cause of the problem, so any advice would be appreciated.