I encounters the same kind of errors since yesterday on Ubuntu 10.10: random (and frequent) freeze. Sometimes, network access from a remote computer is still possible, but sometimes everything is locked down and I have to power-cycle the machine.
I tried to put 260.19.44 driver, I removed sync to vblank from compiz and tried with and without syn to vblank from nvidia settings, nothing works.
With the original drivers from Ubuntu (260.19.06), I got thousands of messages in /var:log/messages like:
Apr 15 20:53:04 lehrin kernel: [44982.434618] NVRM: os_pci_int_handle: invalid context!
Apr 15 20:53:04 lehrin kernel: [44982.434660] NVRM: os_pci_init_handlet_handle: invalid context!
Apr 15 20:53:04 lehrin kernel: [44982.434686] NVRM: os_pci_init_ht_handle: invalid context!
Apr 15 20:53:04 lehrin kernel: [44982.434736] NVRM: os_pct_handle: invalid context!
Apr 15 20:53:04 lehrin kernel: [44982.434766] NVRM: os_pct_handle: invalid context!
Apr 15 20:53:04 lehrin kernel: [44982.434807] NVRM: os_pci_int_handle: invalid context!
The "os_pci_init_handle: invalid context!" was the most repeated one (more than 120000 occurrences).
Then I got a one minute gap without any messages in /var/log/messages followed by kernel errors (see attached nvidia-bug-report.gz, where I have removed many of the repeated messages to fit the forum upload limit).
After manual power-cycling and reboot, I could work for about 10 minutes and then got another freeze, but this time without the thousands of messages.
Note that in the attached bug report, some files reference the 260.19.44 driver I installed today (April 16), but the /var/log/message that was included is from yesterday night (April 15, at about 21:00 UTC) when I had 260.19.06 installed. From a user point of view, I don't see any differences between the two drivers and get the same behavior, so I guess this mismatch is not relevant.