It couldn't be the gas because the way you described your engine when it was running was fine and if it was the gas the engine wouldn't be running fine. It could be the fuel Injection system, it might have been set to European specs when it should be set to american specs. This car probly went through so many shade tree mechanic and everything is out of specs. You are an old mercedes driver probly you will notice If the transmission shifts to fast up in gears than you have the same symptoms.