Occasionally, I have to turn the key several times before the engine starts in my 98 E320. I hear the starter turn for a second or so but then it stops. There is no starter sound even when I hold the key in the last position although for some strange reason my RPM gage keeps jumping back and forth. The only way I can start the car is by turning the key back to the first ignition position and retrying. Sometimes, I have to do this several times before the engine starts.

Any idea what is causing this problem?