I have a 1991 190e mercedes benz that I have been trying to work on for a while now. The problem is that it will crank, but only run for about a minute and a half to 2 minutes, idles down, revs back up then shuts off! It does this over and over. I have changed the fuel pump, fuel filter, spark plugs and spark plug wires and now it runs a minute longer but does the same! Can someone help me out on what to do next?