Before you can take it to a MB mechanic try this "field diagnosis". Turn the key to the on position (all the lights should be on) but do not start the car. You should also hear the fuel pump(s) working. Leave it like that for a few seconds. Turn the key off and then, start the car. If the problem is better or gone, you are loosing fuel pressure somewhere. Take it to your favorite mechanic for proper diagnosis/repair.
