Lemmon,
You may have one that drifts. It happens. You can send it to RCBS for repair. Their customer service is excellent. But before you do that, try several things:
First make sure that no computers or fluorescent lights are operating anywhere near the scale. The interference can mess them up.
Second, take a computer interruptible power supply and plug that into the wall then the scale into it to get the internal filtering. This works even if the unit's battery is long dead. You just want the filter. Fluorescent lamps and computers not only radiate interference through space (the reason for the first step), they can put interference into an AC line, and that's what the second step addresses.
Third, leave it running fifteen or twenty minutes before using it to make sure it has warmed up.
Fourth, use a cardboard box on its side as a draft shield and make sure it is where the temperature is steady. You'd be surprised how much forced air from a heating ducts can upset a measurement this delicate.
If you don't get satisfaction from RCBS, take a look at the $75 battery-powered
scale the Brian Enos is selling. I called him to inquire about them and he told me they're stable and he's never had one returned. Batteries may seem like a nuisance, but they give you immunity from line noise so you don't need the computer UPS.