You say seems, either it is unstable or its not. Set the pitch so the edge platter dots are stationary (not necassarily at zero) and see if it starts movin back or forwards, if its still,slow the deck with your finger, does it return to stationary dots? If it does then all is fine. You shouldnt be blaming a bad mix on your decks, esp. when they are 1210s lol. But seriously, if its not keeping a steady pitch you can get it tuned for about £40 a deck at a Technics dealer (dont quote me on that price tho) :O)
Hope this helps
Jex
August 2008