Have you cleaned the heads and capstans again since you first got it?
Mine was doing something similar, sounding muddy and then okay again
after stopping and starting. It was also slightly chewing the tape.
Once I gave it a good cleaning with 99% anhydrous alcohol, it was
fine. I was surprised at how much brown stuff came off on the swabs,
too, considering I'd only played a few tapes in it before the problem
began (they were pretty old, though). So now I clean the heads after
every 2 or 3 tapes, and each time, a little bit of oxide shows up on
the swab. It's not really noticeable if you just look inside the
machine, but it's there, and it builds up.