[ben]

Look carefully at the tape. Are the edges 'wavy'? If so, it has been deformed through playback on a deck with excess take up torque, and will never wind evenly.

When re-shelling, do not be tempted to remove the liner sheets inside the cassette as others have suggested. These are made of specially lubricated material play an absolutely fundamental role in preventing the tape layers jamming against the plastic shell and causing flutter.

Winding end to end also is of fairly limited use. All except the best decks have insufficient back tension in thes emodes and winding tends to be uneven. Playing a tape right through in one pass is the only way to pack it properly, as the take up spool is under tension to the pinch roller/capstan.

In this case, apart from the shell problem suggested earlier, I suspect the tape could have deteriorated or the machine's pinch roller may be dirty or slightly hard. Portable equipment especially used very small roller s so even a bit of dirt can cause wow. I would hold off doing anything until you've tried it on other decks.