GTX260 - Problems switching resolutions

I recently bought a new monitor -- an LG IPS231 -- and it's having some strange issues switching resolutions. Pretty much only 1920x1080, the monitor's native resolution, works properly. If I have a game that runs at, say, 1280x720, I end up with an image that fills the monitor vertically but is squished horizontally with huge black bars on either side. The strangest thing is that if I go into the monitor's menu while at 720p, it reports the input resolution as 1280x1024. This happens in every single game I've tried.

Does anyone know what could cause this, or even if this is likely a video card problem or monitor problem?