my 2000 SLK230 has been randomly shutting off, and just as randomly not starting on occasssion. it has quit while i was driving around 80mph, 40 mph and idling at a stop light. It will start fine fopr a week and then not start. after i keeep cranking (it doesn't actually crank, nothing happens) for a while it may start. Then an hour later it may be fine. I have had the air sensor and crankshaft position sensor replaced. Do you think it could be the fuel relay, or fuel pump?