My sister called a couple evenings ago and said "I think my DVD-ROM is going bad! Videos are so choppy and jerky, they're no fun to watch". This was caused by her secondary IDE channel reverting back to PIO mode, an older method for accessing drives - and too slow for DVDs. To fix, we uninstalled the channel, redetected it, enabled DMA mode, and then rebooted. See this article on DMA reverts to PIO.