I get very similar issues with my Geforce 6800 Ultra, beginning around three months after I acquired it. It's really starting to bother me now, with artifacts stretching from HUD elements, sprites and the ingame graphics themselves either horizontally or vertically, though most often radiating in a spiky halo toward the center of the screen.
I get minor issues with this in Halflife 2 mods, more major issues in Halflife 1 mods, severe issues in any game where I turn on HDR or Bloom effects and when it does appear, anywhere from five minutes to an hour into my game, performance then degrades over the next few minutes until the game crashes, by which point the view is almost completely obscured by these stretching textures and effects, depending upon where I look.
The stretches actually grow across the screen in real time depending upon my view.
I'm not tech-educated, I have no unusual settings or programs related to graphics and I am not overclocking.
Average temperatures on the card (i've heard a lot over overheating due to a loose fan on the 6800s) are an ambient of 45-55oC and a Core temperature of 65-82, depending on whether I'm on my desktop or running a graphics intensive game.
These temperatures are in the middle of a southern-uk winter, with heavy snowfall and below zero indoor temperatures. It should not be overheating, it should be turning into an icicle! I certainly am o.o
I'm also running the latest drivers.
Any help would be appreciated.