Ahh, yeah then I think your only option is to try to see if boiling it will work or replacing the belt. You can try looking up the service manual for your boombox online to see if it has the belt size or try using the PRB cross reference for it. If you measure the belt you might want to subtract around 10% from the overall length to account for stretching over the years.

BTW if you're interested, I posted a couple of pictures of my cassette decks on the off topic gallery.

I believe there are a few sellers online. Vintage Electroncics and A1 Parts list them.

Yeah uneven thickness, improper belt size, and sometimes even too thick of a belt can cause excess W&F. Granted the problem is seen more in the more complicated mechanisms like the ones that use dual capstans. 

Or often contaminated or hardened pinch rollers are another cause.

The dual capstan mechanisms I've mentioned are unidirectional and keep both capstans engaged while playing.

They are both rotating, but actually only one of them (the one pulling the tape from the heads in the selected direction) is really engaged by pressing the tape to it by the corresponding pinch roller. The other is just spinning freely in the air.

They are both rotating, but actually only one of them (the one pulling the tape from the heads in the selected direction) is really engaged by pressing the tape to it by the corresponding pinch roller. The other is just spinning freely in the air.

No, that is only for the auto reverse decks, I have a unidirectional dual capstan deck and both are engaged simultaneously while playing. If you read my entire comment, I differentiate between the two different types of mechanisms with dual capstans.

Or often contaminated or hardened pinch rollers are another cause.

Thanks for reminding me to clean them when I take it apart to boil the belt! I also have to clean the capstans where they touch the pinch rollers because they and the pinch rollers have this weird grime on them.