So, the game only runs at a refresh rate of 24hz while in 1920x1080, other resolutions will cause the game to either run at 50hz or 60hz (1280x720 is 50hz and 1366x768 is 60hz, for example). This is strange because all of the resolutions in the list state that they run the game at 60hz. Forcing vsync on doesn't change anything. Any idea why the game would be doing this? It's completely unplayable at 24hz.

What is your monitor's native resolution?

1920x1080.

Hmm, maybe you can force the graphics driver to use e.g. 60 Hz.. The Catalyst Control Center for example has an option to do so for programs running in fullscreen: Display Options => 3d Refresh Rate Override

Nvidia and Intel drivers might offer something similar.

Wonder if your monitor (or GPU) is on some funny TV/movie settings?
1080p@24Hz is full-hd movies and the other two are typical TV/DVD outputs.

Negative. The monitor is running at it's standard 60hz refresh rate when on the desktop and running any other game (all in 1920x1080). This is unique to NWN1 for this setup. :\

I'm going to try to override it as DeMignon suggested and I'll post back with results later.

If possible, when in game, Alt-Tab out to your desktop...

If you are using an AMD/ATi gpu, and the Catalyst drivers, you can right-click on the desktop, select "Screen Resolution", and a new box will open with the words "Advanced Settings" in blue lettering near the lower right corner ControlPanel\Screen Resolution box. Select Advanced Settings with a left-click, and a new box with several tabs will open, with the "Adapter" tab already open. The button you want is at the bottom left corner of the adapter tab, called "List All Modes." Left Click List All Modes and you can then select the refresh rate and resolution you want. Just for fun I checked my Catalyst drivers (13.12WHQL) for 1080P and found that 1920x1080 @25Hz Interlaced was indeed a selectable and supported resolution with my GPU and monitor driver.

If the game is for some reason causing the Windows refresh itself to change, then the above steps should fix the problem as a temp workaround until you can get to the bottom of this. However, if this is caused by some kind of obscure driver bug in your graphics device and the way it interacts with 3d-game refresh-rate calls, then the above won't work, unfortunately, as the game won't be changing the Windows refresh rate--it's just changing its own refresh rate independently.

In any event I highly recommend at least a reinstall of your graphics drivers and the game itself, as this *should* correct the problem. If not, though, then definitely *change* your GPU driver, even if you have to regress a version--which definitely should fix this problem.

Also just for fun, I just ran my own version of NWN Diamond, and I'd forgotten how buggy it can be when it wants to...;) I'll get back to this post...

I downloaded the 2.x installer version from Gog just now, ran the game--it's one of the few I have that won't run in Win8.1x64 unless it's in compatibility mode--I found that XPsvcpk2/3 mode seemed to run the game the best. No problems with 25Hz, however.