1999 Mercedes Benz 320e Car won't start periodically

Electrical problem
1999 Mercedes Benz 320e 6 cyl All Wheel Drive Automatic 123000 miles

At times, when I engage the starter with the key, the engine will turn over normally but won't start.I checked the spark when this happens and their is "no" spark. Sometimes the starter keeps going enven when I stop turning the key. After a while, sometimes as long as a half hour the car starts normally however this condition is occuring more often over time