Relax and spin round circles
You need to let go of the washing machine once you get going - bend your elbows, relax your shoulders, your upper body being stiff is probably partof the wobbliness. Second the rollers teach you to be smooth with your pedal stroke - or you wobble! so, think about round circles, being as smooth with your pedal stroke as possible - some people say think about wiping your feet off on an entry way doormat as you are going thru the bottom of the pedal stroke, this helps make your pedal stroke round - the other tip is to pedal over the top of the pedal stroke.