Changing the Resolution in 10.1


I am on 10.1. i have a Samsung 950p 19" Monitor connected to my G4/400. I can work fine with 1280 at 85hz but when i change to 1600x1200 it automatically changes to 85 hz which my monitor does not support. i have had to reinstall the system to get it to work as once it goes to 85hz I cant do anything. is there any way I can select 75 hz. I cant even play Quake 3 at 1600x1200.