Monitor Resolution Problem

I have two monitors hooked up to my pc. On every other boot the main monitor's resolution changes from 1920x1200 to 1600x1200. The second monitor remains 1280x1024. I am running windows Vista Home Premium 64bit SP 2, and when I go to display settings when it is at 1600x1200 I am not able to move the slider up to anything higher. Also, when it is at 1600x1200 I am sometimes able to change it to 1920x1200, but above I classified this being the 1920x1200 resolution on start-up. Display settings classifies both my monitors as Generic non-PnP Monitors. Any help or suggestions would be greatly appreciated.