300 Dies Repeatedly -- HELP!

1989 300E(6 cylinder) Mercedes, 57,000 miles runs smoothly, accelerates great, doesn't smoke, etc., 23 MPG. However, occasionally it cranks and runs perfectly for approximately 2 seconds and suddenly the engine stops as if ignition turned off. Repeat cranks and runs smoothly for 2-3 seconds and then dies. This will repeat itself until the battery is dead. However, if the negative battery cable is removed for a few seconds and then re-applied, the car immediately will crank and run for a variable period of miles (from 70-500 miles) before the above failure repeats itself. This has occurred 12-15 times in the past 4000 miles. I have replaced a voltage relay part #210 540 38 45 (Seimens SWK1763, 10A, 12V) but nothing changed. I have discussed problem with 2 different Mercedes Benz mechanics and they seemed stumped but could figure it out if I brought the car in when the problem occurred.