How old is your powerbook?
Mine started doing this a month or two back, and I bought it last november. I had to take the book in for a hinge replacement and they told me that they'd replaced the drive chassis. After that, everything worked normally again.
I think that it's possible for the drive to become misalligned, and this would cause problems. I have used the same brand of blank cd's (compusa $10/100) for the past year, so I knew it wasn't a media problem.
If you're still under applecare, take it in and get it looked at. If you're not, then I hope someone else may be able to provide some info on checking the drive allignment.
By the way, although I travel with my powerbook regularly, it is thoroughly babied and never bounced. I hope this doesn't become a feature of the book.
Hope this helps,
J.