Originally Posted by ShannonM

First thing to check is the tightness of the shift levers on their bosses. The friction that makes friction shift levers friction shift levers is the friction between the outer plate, (underneath, and often part of, the mounting screw,) and the inner plate, (sits between the inside of the lever and the frame boss or clamp band that it mounts to.)

This friction is what resists the force of the return spring in the rear derailleur. If the levers are loose, the spring will drive the derailleur towards the high limit screw, especially when the rider exerts a significant side load on the derailleur, such as by standing on the pedals.

If you can't get the levers tight enough to hold the gear and still be easy enough to move, then probably there's a washer or other small part inside the lever that's worn down past it's lifespan. At which point, you'll need new levers. If you can get them set right, and then they loosen up over the course of a few rides, try coating the threads of the mounting screws with nail polish or beeswax. (Loctite is too much here, IMHO.)

--Shannon