It could be copy protection on the discs. A normal CD player has error correction circuits so that if a small portion of data is missing it gets interpolated from the previous and next portions of data. With a CD reader in the computer there is no error correction circuitry as that might mean that data discs with a glitch on them would also get 'error corrected' and end up with a corrupt file. The CD reader in a computer keeps re-reading a faulty section until it either gets a good reading or gives a error report or just goes into a loop of trying to read that section. If the CD's play correctly in a music CD player then that is probably the cause.