The nvidia driver is probably using EDID to get your monitor's capabilities (i.e. refresh rates, display size, etc.) You can check this by looking in the Xorg.log file for messages about EDID.
If you look in the documentation that came with the nvidia driver you will see that there is an option called "IgnoreEDID" that you can use to bypass the driver autodetecting your monitor's capabilities. I think you do this by adding the following to your Monitor section in the Xorg config file:
Option "IgnoreEDID" "True"