Sounds like the map CD may be on the way out.

First try cleaning it (household window cleaner) and see it that gets it to read.

If not, download a copy of "imgburn", stick the disc into your computer, rip it to an ISO file and then burn that to a new disc. If you get catastrophic read errors trying to do that, then the thing's U/S. Alfas may well be able to sell you a replacement, or the "usual suspects" for sources of hooky material on the internet may well have a downloadable version.