I have same problem (symptoms, logs, xorg and nvidia are equal). Linux b-s-a 2.6.31-15-generic #50-Ubuntu SMP Tue Nov 10 14:53:52 UTC 2009 x86_64 GNU/Linux AMD Athlon(tm) 64 X2 Dual Core Processor 4800+
I have same problem (symptoms, logs, xorg and nvidia are equal).
Linux b-s-a 2.6.31-15-generic #50-Ubuntu SMP Tue Nov 10 14:53:52 UTC 2009 x86_64 GNU/Linux
AMD Athlon(tm) 64 X2 Dual Core Processor 4800+