correct directives from the previous 2 posters. Use a bubble level to make sure the table is level. Also, make sure the arm is perpedicular to the table itself. If you have an anti-skating adjustment it could be set very loose in the sense of it trying to counteract the rotational forces exhibited by the TT. You want the anti-skating force exerted on the arm to be pushing towards the center of the platter. Also, check your down force adjustment to make sure that the cartridge is sitting on the surface with not too little pressure. Now, here is something that effected my setup in the same way. I had rewired my tonearm and after seeing the same phenomenon as you, I realized that the wires were oriented in a way as to exert some force on the tonearm. The tonearm would just lightly move as you described until I re-adjusted the positioning of the wires in relation to the arm mechanism. Good luck.
