Just a thought.... the tape is pushed against a felt pad inside the shell during playback/record only but these pads can deteriorate over time. The pads collect any deposits or muck off the back of the tape.
Are you using a new felt pad when you transfer it to a new shell? Even unused pads can deteriorate in storage!
A noble suggestion, but 1. Yes, it's a new pad in a new shell. 2. Old and new pads are in fine shape.
Mike -- mrichter@xxxxxxx http://www.mrichter.com/