NAK 670ZX play problem, an easy fix?

Hello, everyone. It looks like I may have gotten burned on a Nakamichi 670ZX cassette deck I bought on eBay. The unit had been represented to me as working for the previous owner, as recently as last September. I tested the unit upon its arrival to discover that the Play function is not working. FF , Pause, RW, and VU meters seem OK. However, the Play runs the tape at a very high speed when played from the beginning of the tape. As the tape progresses, the Play speeds up and slows down in ever-widening whirring spurts. When the middle of the tape is reached, the tape simply shifts to Stop mode. Also, a brief whirring, squeaking mechanical noise comes from inside the deck when the tape is manually stopped at any point in the play and also when it stops by itself . I'm not a tech, so I'm anticipating this could be a costly and prohibitive repair, unless it's a simple and quick fix I can do myself. Does anyone out there have any familiarity with this type of problem and this deck? Comments and suggestions will be appreciated. Thanks.
See the link below. Almost all older Naks have some sort of drive problems.
OK, thanks. I appreciate the info.
You're not the only one.. I had a similiar experience purchasing a CR-3A for a very low price on ebay.. worked for about a month and then the drive quit. I gave it away.
On many units you have to do the "gear drive upgrade" and that will fix the drive problems.. I had this done to my BX-300, and my Special Edition Cassette Deck One (bought here on 'gon) already has the gear drive and has had no problems. Good luck finding someone who can perform the operation or try Electronics Service Labs in CT (but I hear they're not cheap).