Nothing wrong with that graphics card at all. It's still a good card, Fermi generation, good enough for Manifold 9 (and Viewer) as well as Manifold 8. It does have only 1 GB of graphics RAM though...
Let's do some maths, and make some wild guesses about how things work.
You say you consistently see the corruption making an F6 image over 10000x10000 pixels.
First obvious point: that requires rendering a much larger image than your monitor can display at once.
But I assume that Manifold still asks the graphics driver, and probably the graphics card, to do the rendering--otherwise we would not consistently see a match between on-screen display and an F6 rendering. (I know, that is begging the question--you are not seeing a consistent match, but corruption. But let's come at that in another way below.)
How much memory is needed to render an image of this size? 10000x10000 pixels x 3 bytes (24 bit) RGB gives 2,400,000,000 bytes, divide by 1024^3, -> roughly 2.24 GB. So already, a 10000x10000 pixel image needs much more than 1 GB of graphics RAM, it won't all fit on the card. For 15000x15000 the requirement is about 5 GB, and for 20000x20000 about 9 GB.
So where does the extra RAM come from? It depends: it could be that image is rendered all in one pass, or it might be rendered in tiles which are then combined. I don't know which.
Let's say (really a guess) that Manifold simply asks the graphics driver to render an image of X x Y pixels, and leaves it up to the driver to find the memory to write output to. Let's guess again that the graphics driver first uses up any spare graphics RAM (in your case, 1 GB less whatever is already used for display), then spills to system RAM (up to 32 GB, less what is already used by Windows). And that the driver renders from top left to bottom right. Hopefully that's all at least plausible.
In your case, when rendering a large image, you consistently get clean output in the top left, corrupt display elsewhere.
So it could be (can't put it more strongly than that, even this is tenuous) that your video card and graphics RAM are absolutely fine, explaining why the first part of the image to be rendered is clean; but that you have one or more sticks of faulty system RAM, meaning that later parts to be rendered are corrupted.
So I would run a thorough test of all your system memory. The good thing is that it is very easy to test RAM, and replacement RAM is cheap. The bad thing is that if you have faulty RAM, it may have been compromising your data generally, not just image rendering.
I would recommend Passmark Burn-in-Test for testing RAM but there are other good options.