Reflection Rendering Bug (GF 600 Series Only?)

I put a GF 680 in this computer several months ago and since then I haven't been getting reflections from "fullReflect" water objects. Specifically the reflections will cut out at any resolution above 1360x768 (they stop rendering if you switch to 1366x768).

At 1360x768 everything looks normal:

But at 1366x768 or higher the world reflections stop, leaving only cube map reflections (in this case I've left the default solid black cubemap to make the difference more obvious):


Same results with 1.2 and MIT 3.0, even with the pre-built demo executables. It's also persisted through several months of driver updates. That leaves the video card as the most likely suspect (or Windows 8, which I installed at the same time).

I'm hoping that someone else here has a 600-series card and can check their water reflections for this issue. Someone with another card confirming that their reflections work fine at the same resolutions (especially on Win8) would also be helpful.