My 86 560sel sputters as i first step on the gas. At about 1500 rpm the engine picks up and runs like the beast its supposed to be. When i come to a complete stop and the car is still in gear the engine misses constantly. If i just put it in neutral it revs a little and runs smooth again. I just bought this car for next to nothing and put heat in the gas tank, along with a good amount of the recommended "premium" fuel, and it seems to have helped a little bit so far. The previous owner said he checked the spark plugs and they looked pretty bad, and he couldn't remember the last time maintenance was done on the car. I am not new to auto mechanics but am new to this car. Any suggestions on where to start?