Powerbook combo drive not accepting discs

[dave vauxhall]

My G4 Powerbook 550 (Titanium, bought in December 2001) is not acccepting discs in its slot loading combo drive. Perhaps one in twenty times the disc is read and mounts, but most times the disc is drawn into the drive, and then ejected a few seconds later. The same disc might be rejected ten times, and then accepted the next. Discs that are rejected are ejected in less time than it takes for a disc that is accepted to mount - it seems like a mechanical problem where the drive is ejecting the disc before it even tries to read it.

I have tried resetting the Power Management Unit on the Powerbook, which hasn't improved the situation.

Does anyone know of any firmware or software fixes that I could try to avoid having to send the computer off for repair?

Any suggestions gratefully received!

Dave