If you have the New F-1 (sometimes called the "F-1N" not to be confused with the second version, the "F-1n") then it sounds like you may have the focusing screen upside down. I didn't know that it was possible to install it upside down, and I'm not about to try it on mine, but the matte/grooved side should be down with the black edge under the tab at the front of the camera and the chrome edge on top toward the back of the camera.

If that's not it, it's something like a mirror alignment problem. Make sure there's nothing obviously preventing the mirror to return to the normal viewing position.