Yeah, your computer is having trouble writing the log file reliably, presumably because of the freeze. However, there are a couple of logs with some interesting tidbits:
(II) NVIDIA(0): Initialized AGP GART.
(WW) NVIDIA(0): WAIT (2, 6, 0x8000, 0x00000000, 0x00000624)
(WW) NVIDIA(0): WAIT (1, 6, 0x8000, 0x00000000, 0x00000624)
(II) NVIDIA(0): Setting mode "nvidia-auto-select"
(WW) NVIDIA(0): WAIT (2, 1, 0x8000, 0x00000000, 0x000006bc)
(WW) NVIDIA(0): WAIT (1, 1, 0x8000, 0x00000000, 0x000006bc)
This indicates that the driver is having trouble communicating with the GPU reliably, and the GPU hanging or responding with garbage. AGP systems are notoriously buggy, so you might have to do some experimentation. A good place to start is by reducing the AGP rate from 8X to 4X or even 1X. Also, you could try disabling the kernel's built-in agpgart driver and using nvagp instead. There's a little more information available here: ftp://download.nvidia.com/XFree86/Li...hapter-12.html
Did this regress recently? Was it working reliably before you installed Xubuntu 9.10?