When I use mplayer (i.e. play a movie with it) the system will sporadically freeze completely, for around 13 seconds. Everything halts as if the system is having a kernel panic or worse -- even the clock halts.
Around 13 seconds pass, everything returns to normal (the clock then jumps 13 seconds ahead, as well), and from the kernel pool log (dmesg) I observe this:

NVRM: Xid (0000:02:00): 8, Channel 00000004
This seems to happen at sporadic time intervals, but can happen as often as every two minutes at "worst", making watching a movie clip the most annoying experience you can imagine.

There are no errors from mplayer.

What the heck is this? I have tried to update the graphics drivers to the latest version, to no avail. I also have no idea how this issue started in the first place.

I use Slackware 13.1.0 on a fast x86 (4.1 GHz Core i7, 6 GiB RAM, SSD, GTX480, etc.). There are no other known issues with the system. I don't use any sort of package managers or automatic updates. I've been trying to figure out what I did that caused this issue to start in the first place, but I have no idea.

This issue happens regardless of whether the media played is physically local on the computer, or if it is on a network.