Screen problems

I have installed 8.04 on a Sony VAIO desktop with a Gateway screen with a native resolution of 1680x1050.  Everything works well at first, with the screen at the proper resolution. If I reboot the computer, it comes up at 960x600 resolution, and that's the max allowed if I try to change the screen's resolution.  I can't find any way to designate the screen in any hardware setup.  It seems that I was able to do that with Fedora 9, but I can't remember how I did it there.  The 960x600 is impossible to use, and the only way I can get it back to native resolution is to do a complete reinstall.  Then if the computer is shut down for any reason it reverts back to 960x600.  Utter frustration!  Can anyone offer a suggestion?