Well, it likely affects the GUI mostly, because that stuff is where any load would be put on the videocard. The TTY's or console use minimal computing power.
You say you've cleaned/dusted the card. I will presume then that the fan on the card is clean and spins freely, and that you have observed that it actually spins and works fine when the card is running?
And how about the rest of the computer, as far as being clean inside, dust free, including the power supply ( I recently took my power supply out and apart for the first time ever on my 3 yr old machine, and WOW! I could barely identify any of the components inside there :P)..
The version of Xorg (7.1) you are using, is that a stable, reliable version? Do you know if this problem had occurred before or after any particular software upgrades or changes?
Are you using the driver from www.nvidia.com
or a linux open-source version?
I'm undecided as to whether it sounds more like a hardware issue, or a software issue. The lines appearing is definitely weird, but the ICONS DISAPPEARING makes me lean towards a software issue.. I'm uncertain though.
Now, the problem occurs you say after like an hour of operation, but instantly goes away if you click out to a console and back in? And then, does it take another hour to return or only like 10 mins? If you keep using the machine, does it continue to get worse? What about if you run soemthing really graphics-intensive, does that make the issue happen sooner?
I'm not sure if nVidia offers any hardware diagnostic tools, but they *do* have some developer tools and other odds and ends.. Maybe worth checking out the nvidia users Forum and browsing the site for similar issues.
Finally, I have the exact same card as you, about 3 years old. My kernel is 2.6.20, Slackware 11, X version 6.9, using the nvidia closed-source driver #9631 from nvidia.com, and have no problems whatsoever..
Happy to try to help you figure this out, but not sure if I can come up with a final answer... If you could update as to the questions I've posted here, we'll see where it goes