Comment 2 for bug 517222

This is a very common, and known issue with the ATI video driver. From what I understand, is basically a whole set of Radeon cards (including the ATI Radeon Mobility X1600) have poor performance and various bugs with X drivers (both open source & proprietary). It's a known issue, one that doesn't seem to have a solution beyond what you've already discovered (nomodeset). My suggestion, try another driver and see if it works better, or live with nomodeset.