HELP!!!

I have a 1995 E320 sedan, 120,000 miles. 3.2L inline six. Dual fuel pumps. When idling, in or out of gear, A/C on or off, the engine lopes, almost dies, then catches itself and kicks back up.

If I am in gear, 10-15mph and higher, everything is fine. Good throttle response/acceleration etc.
No change is gas mileage. No problems with engine temperature. Changed fuel filter. No help.

Any ideas? Thanks

Lost in NM.

One more thing. If I put a bottle of Valvoline once-a-year fuel injector cleaner, the car runs smooth as silk,.....until that tank of gas is gone, then back to the old bump and grind.