I had this also and my refresh rate would change to 60Hz also. Right click desktop, properties, settings, I can see "Dell P991 on GeForce Nvidia GeForce4 Ti4400" click on the drop down menu and you see default monitor on GeForce etc. Disable the default monitor by unticking "exists in all hardware" I am 99.9% sure this is the cure, it is so long since I came across it. Let me know if it cures it. Wossie.