400E wierd cold idle
My 92 400E, with about 130K, idles rough when cold especially after sitting for a few days. While it's in idle, if you accelerate hard and let go, 1) there is a wierd "backfire" sound from the exhaust, and 2) when the RPM's come back down they fall below normal and almost kill the car, including any interior or exterior lights that are on. After this happens the RPM's go back up to normal but still idles a little rough. This continues until the car is warm. When it is warm it does not have this problem. I keep the car well maintenanced and did a full tune up about a year ago (plugs, wires, caps, rotors, etc.) Anyone have any clue what could be causing this? Thanks in advance!