Just so we're all clear on everything, the focus locking screws are the metal ones (about an inch across) right behind the focusing knobs, like an inch and a half back? Should be two of them, one on each side, pointing upward. Then there are the extension bed locking screws, these have plastic grips, and there is one on each side of the rear bed pointing outward. Finally, there is the single rear frame locking screw that is made of brass, has a plastic grip, and points straight backwards. Its the one that secures the rear bed to the front frame and standard.
In order to focus, the focus locking screws need, obviously, to be loose.
The rear frame locking screw should be good and tight any time the camera is unfolded into working position. Make sure there isn't any grit or anything between the surfaces that this screw clamps together, as they could potentially throw something out of alignment.
The extension bed locking screws should also be tightened whenever you aren't actually moving the bed in or out. The bed can also be completely removed from the camera, and reinserted in either direction. Try inserting it both ways, because for some reason my camera only focuses smoothly with the bed inserted one way, but not the other. Yours might share the same kink.
Also be warned that the focus locking screws cause a very slight tilt to the rear standard. You can really only see it one the bubble levels, but its there.
These cameras are pretty darned easy to take apart. What I suggest you do is slowly disassemble each component of the camera, one at a time, clean everything really well and rub all the sliding surfaces with medium and fine steel wool. Then, reassemble and move on to the next section of the camera. Worked wonders for mine.