[Pezman]

What program you using to burn your cd-r's?

Two thoughts come to mind. 1. You're burning the cds at too fast (ie 52x or 48x)
I recommend burning cd's at between 24x-40x oversampling. Any faster and you could be getting artifacting on the discs.
2. Your burner could be to blame. When you are writing the discs, are you multi-sessioning the discs? Sometimes if you have a 650mb disc and you only put 500 mb on it, the program doesn't finalize the disc (*which means no further writing is allowed). You should always finalize a cd-r as open session discs tend to have problems with non-cpu readers.

Lastly, it could be the media you are burning with. What brand name are you using? Is it a quality brand? Is it specific to music cd-r's? Those would be the first things I would diagnose. Good luck!