I see that you've got v1.1 set to 16-bit. Have you tried setting v2.6 to 16-bit? Some cards may smooth out the aliasing when dithering 16-bit color.
Edgar sent me his test version yesterday, and it did not seem to make any difference on my system, though it did on all three of his. So, I got out the old code and did some experiments myself. There are several places in the program where filtering is set, and if I replace all of them with Anisotropic filtering, and set MaxAnisotropy to 2, then it definitely smooths out the background when stretching it to a hi-res LCD monitor. Too much technical info ?
We have to run some tests to make sure this didn't break anything else, then we'll let you download and try out the fix.