I feel your pain, Tofu. I own
a Yamaha MT-50 4-track and in January, one of the tracks (the 4th one) started to loose its ability to over-dub. I started hearing 'ghosts' or shadows of previous recordings on the track every time I tried to record over it. I asked the same questions as Dom did above and determined that it was time to say goodbye to my loveable, hugable, analog four track.
...So 9 months later; I have just completed a new, digital setup. I bought a Fostex FD-8 digital multi-tracker, a new killer PC, and a pro sound card as well as million other accessories,cables,monitors, etc... Now I go 8-tracks digital on the mult-tracker and mix down to my PC. Then I touch up the mix using mastering software and can burn to CD. Don't get me wrong, I know this is a tremendous investment, but the benefits, flexability and quality you get out of the digital recording process is incredible.
Sorry for this digital blip in this all-analog post site; but I once was terrified of digital and stayed with my 4-track for fear of the investment and complexity, but now I'm one of the many converted.
Bill