I've seen a couple of folks mentioned checking the copper eddy current damper plate (aluminum on my Ohaus, but everyone is saying copper, and I don't have an RCBS version handy to check). This plate swings through a slot between two opposite permanent magnet poles. That causes the moving plate to cut through the magnetic lines of force, generating eddy currents by induction. The solid metal plate then also shorts out that induced current, loading the "generator" down, making it drag. This is why it damps out the beam swing much more quickly than would be the case if it were simply a free balance.
If that plate were dirty or bent, as mentioned previously, it would rub the magnets and cause the observed irregularity. However, I find it much more common that a splinter of steel wool, a staple, or some iron filing dust has been caught by the magnets and drags on the damper plate, but does not come out with it.
Lift out the beam and shine a light down between the magnets to see if anything is there. If so, try cleaning it away by sliding some tape into the slot and up against the magnet to grab the errant steel or iron. Do this on both sides until it looks clear.
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