Why won't my cassette tape play all way through?

I have just bought second hand what looks like a perfect condition EMI pre-recorded cassette tape with music. It starts playing ok but when it gets near the middle of each side, the sound stats to distort and eventuially the tape grinds to a halt. I can fast forward or rewind but clearly the player motor is having a hard time turnignthe spools. I have ff'd and rewound non-stop all the way through several times, shaken the cassette - but to no avail - it will not play beyond about 8 minutes into either side. Very frustrating. The tape itself looks ok - not crinkled or bunched up at all. The casette body looks like new (but I guess must be 10 years plus old). It was bought on e-bay and I have e-mailed the seller about the problem; no info yet on how it was stored etc. It is not a particularly valuable cassette but is a rare version of an album with additional tracks not available elesewhere. So I am really keen to make it playable. Is there a way of fixing the cassette so it can play all the way through without taxing the motor? Grateful for any tips!

Mark