I've seen problems similar to this when RenderAccel is on (which is known to be broken after 3123). RenderAccel will also occasionally cause the X process to hardlock and eat 100% CPU.
Unfortunately this isn't mentioned in the man page, but if you check the ChangeLog after 3123 there is a small mention of the issue.
Nvidia - Can we get this fixed guys? I'm almost tempted to revert to 3123 because of the boost in 2D speed...