bizarre monitor resolution problem

All of a sudden my monitor is doing strange things. When I am working on the computer it will sometimes, now most of the time, goes from being 800×600 resolution to a resolution that makes the screen appear too big to fit the text on the screen. It will cut off about 1/4 of the information and no
matter what I try to do other than re-boot it will not change back to the
correct resolution. Does anyone know what it might be from? Thanks in
advance.

lisacav@yahoo.com