Sounds like similar problem I had on my 89 300E. Ran good when cold, but as it warmed up would run rough, the economy gauge would stay in the middle or peg to the right. Would only run good at wid open throttle. I cleanded the ICV and throttle body, changed a bunch of vacuum hoses. I tested the O2 sensor by dissconecting the wire under carpet in front of the front passenger seat with a jumper wire. While testing it it ran good when it got warmed up. The connection must have been bad. Connected it back up with a couple of twists and has been running good ever since. Got lucky on that one.