What causes hesitation when i step on the gas pedal for a 2002 mercedes ml 500 The vehicle starts bucking like it is not getting gas.?

A number of things can cause hesitation. There could be water or dirt in the tank. Also, it could be an ignition problem. Your ignition timing may be slightly off and your plugs are firing a split second off top dead center. This would account for the hesitation and subsequent bucking.