I've been having the same problem, I've lost resolutions and refresh rates since upgrading to 9.1. Although it seems I can get them back by changing resolutions and restarting, sometimes. The last time I was only able to keep the usual resolutions for one restart, but the time before that, it lasted 3 or 4. I have no problems if I boot from the 9.0.4 partition so it seems isolated to 9.1, it also stays bad when I reboot with extensions off,
Anyone have any ideas?