Just bought a 1986 300E about a month ago, and the engine appears to be in distress when its idling (its fine once it gets moving, although there's a slight pause when I first hit the gas). I noticed last night that when the engine is stuttering, the oil gauge is at mid-level. When the car gets moving, the gauge goes up to the top (and runs great). Can anybody give me a diagnosis? Cost to repair?

Thanks,
Brendan