well thanks for all of your advice folks!
I guess I will sell my Rollei 2.8E and send the F to Paepke in Duesseldorf, Germany. They seem to offer complete overhauls (which includes the lens). They even replace parts if necessary. Of course they are very expensive.
Meanwhile I've shot one roll of TMX400 with the camera. The results are ok, although I think that the focus is slightly off and the film transport and shutter could use some attention.