Sometimes it's two stops low, sometimes it's OK, right?
Well, being low, it's not light entering the eyepiece around your eye. On a bright day, light intruding around your eye will make the needle on most OM's deflect as much as 2 stops! Shade with your hand, and the needle drops to accurate. So unless it's actually the first 2n that's reading high instead of this one being low....
One thing to check: In a well lit room, center the needle, then move your hand back and forth across the lens to vary the amount of light entering the lens while you watch the needle. The needle should move smoothly up and down as the light varies. If it seems to hang, it could be that a small piece of metallic debris may have "stuck" to the magnet inside the galvanometer. Sometimes a tiny sliver of metal from who-knows-where in the camera will float around until it gets attracted to the magnet at the center of the galvanometer, then it will "stand up", hanging or dragging on the coil frame to which the meter's needle is attached. Then, for whatever reason, the tiny sliver may "lie down", and everything works fine for a while -- until that sliver stands erect again, and the needle stops moving smoothly once more.
If this is the case, the top cover will need to be removed, then the "top hat" over the galvanometer's frame, and the magnet at the center of the galvo will need to be carefully and thoroughly inspected for any debris attached to it. If anything is found adhered to the magnet, a small metal probe can be delicately introduced to attract the metal debris, knowing that the probe will be pulled by the meter's magnet, so hold it firmly! If the metal probe is pulled from your grip, it may damage the meter!
One other problem, though never intermittent, is if one of the tiny coil springs at the top or bottom of the galvanometer "loops" around one of the weighted pins used to balance the coil frame so that the meter reads accurately no matter how it is oriented. If this happens, the "looped" spring will pull the needle away from it's proper location. When this happens, though, the needle will appear to move sluggishly and jerkily, and instead of slowly floating to a stop after varying the light entering the finder, it will stop almost immediately. It is possible to carefully pull the coil spring out from beneath the pin, then press the pin down against the coil frame so that it won't happen again -- but this procedure is even more risky than the one above!
