I also had a Nikon FG with a focus problem once, and it was due to the focus screen being slightly out of alignment. Maybe the build quality is just not that robust? This is a plastic-bodied camera, after all (but with a metal chassis).
My experience was that images were correctly focused on film, if set correctly on the lens, but the viewfinder didn't show the true focus. This was only really noticeable with longer focal lengths.