S430/102k, mi/ Wouldn't start. Told to put car in neutural and 'rock it' 'Rocked' car, put it in park and car started and 'check engine light lit'Prior to the no start, wife was putting middle range gas in car,also gas tank was @1/4. Once car started and home, put HEET(moisture absorbing liquid in tank, and filled car w/ premium gas. Following day car started,no 'Check Engine light'.. Following evening drove car to a concert, while driving in parking lot engine 'quit'. Put car in neutral so it could be pushed into parking slot. Once in parking slot, car put into park, and it started (check engine light on). On way home drove about 2 miles and again engine cut out.Put car in neutral, and tried to start it no luck, 'rocked car, put car in Park and it started. Drove about 12 mi home. Thought problem was bad gas, but seems otherwise. Starter cranks, car doesn't always start.Though *** might be fuel injectors, but they operate ...when car running. No On board testing yet, but spoke to a mechanic, and based on what i told him, he thought it might be a Crank Position Sensor, and Mercedes has had problems with them.