Hmmm.....there is something VERY strange going on, it might be a driver issue, I am going to remove both the monitors driver, and the GFX driver and see if it helps.
Yup, the latest version of the Nvidia drivers seem to mess with widescreen res modes, I don't know why or how to fix it, I had to roll back to the v81 to get them to work.
No, it has specific drivers, but now you mention it, when you install the new Nvidia drivers, the monitor reverts back to plug and play, but even when you reinstall the monitors drivers you don't get the non square modes back.