I have noticed that a lot of the downloaded iso's seem to have a problem after being converted to Live cd's. Sometimes the problem does not get noticed straight away, but manifests as a problem in running some programs and not others. Often the install just won't work.
The best way round this, I have found, is to buy the OS disks. If you get them from a site like On-Disk (
On-Disk.com )you can get copies for as little as £3.00 delivered price. It just takes about 10 days to arrive. At the same time, you are supporting the developers, as they get a percentage of the sale.
I have run 7 different distro's this way and had no problems, but a download of Ubuntu Gutsy Gibbon nearly destroyed my machine.