97 c280 sputters and goes about 10 mph and shakes runs fine in idle

my car has been sputtering when i press the gas in park reverse neutral and drive. when i get it to about 15 rpms it starts to shake and it sounds like its back firing. it sounds like its coming from the exhaust. but its definitaly under the hood. car idles fine. all fluids checked and belts fine. no clues!..:banghead: im banging my head on the wall with this car. what is it n e one know? spark plugs and oil changed recent!