look what resolution your EDID reports.
I run a Toshiba Satellite Pro 6100 with Debian Sarge and XOrg 6.9 from bpo.
The EDID tells that the native resolution of the display would be 969x768 instead of 1024x768.
Older NVIDIA drivers had "IgnoreEDID" to solve this but the new ones not :-(
So I tried and tried for hours (nearly every option I could change in xorg.conf) always ending up in black screens or black borders.
Finally found that solution:
I saved the EDID by nvidia-settings
to a file and edited this.
The horizontal resolution is safed in the bytes with the offset 56(Hex:38) and 58 (the upper four bits) (Hex:3A)
In the original EDIT these bytes were C9 and 31 (0x3C9 = 969)
I changed them to 00 and 41 (0x400 = 1024).
Then I used the Option "CustomEDID" "/path/to/patched/EDID.file" like described in Appendix D of the documentation.
This works for me.
Further information about the structure of the EDID you will find at