I bought a Tascam 564 2 years ago. In the first hour I used it, it froze. I called the technical support and they offered very little help. I took the machine back to the store and exchanged it for another one. 2 days later, I experienced the same problem. I took the second machine back and exchanged it for the Sony.
By the way, I had trouble with the Sony as well. It froze and skipped. I ended up taking it back too. I almost gave up on the MD format. Finally, I spent a little more money and bought the Yamaha MD8.
The MD8 has been a great machine. I've owned it for 2 years now. I've logged hours of recording time. The only problem I've had is a blowm preamp caused by disconnecting the mic cord from channels 1 or 2 while the phantom power is on. It turns out this is a common problem with the MD8 but can obviously be avoided by simply turning off the phantom power before disconnecting the mic cord. Too bad the owners manual does not warn you in advance.
Good luck with your 564. By the way, you might try posting your message on the minidisc forum. You might get more feedback on this issue.
DHN