90 300E 3.0 (well maintained 229k miles) with low, erratic idle. New fuel filter, cap and rotor. Plugs and wires good. Fuel pumps sound good (no buzzing, noise, etc.). Very low idle that "cycles" up to a little higher rpms, then backs down again. When given partial throttle from idle, it wants to sputter (almost backfire). When more throttle applied, it "smooths out" a little, but never fully. At takeoff it will only take partial throttle and picks up speed, but very slowly. When throttle pressed more fully, it doesn't seem to want to increase speed. Once up to speed (50-60 mph), when given full throttle, it kicks down to passing gear, pulls hard and everything is "almost" where it should with the exception of a small, almost undetectable "seat of the pants" miss under full acceleration. I'm stumped and don't want to throw parts at it to figure things out. I'm thinking possible O2 sensor, vacuum issue, etc., but I'm certainly no tech. Pro input is greatly appreciated. Thanks in advance for any help you may provide...