Two suggestions, clean the disks and clean the optical lens on the disk drive.
Eliminate as many scratches and smudges on the disk itself, the little disk cleaning kits at BestBuy and RadioShack and what not actually do some decent work (at least in my experience) but obviously won't clean severe scratches.
Cleaning the optical lens is a little trickier. I would start with the first easy option, open up the CD tray w/o a cd in it and give the insides a good blast with compressed air. Have the Xbox upside down or the CD tray pointing down...anything to make sure the dust doesn't just resettle on your lens.
If that doesnt work, you may want to try digging deeper into the CD drive itself, but let's see if this works first! Let us know.