Assuming the rocking is not from an incorrect saddle height, practice using only one leg at a time when on the trainer- this is the best method I have found for training the brain and muscles to think circles. do a two leg interval and then unclip and do an equal amount of time single legged (hint start with a really simple 1 minute, you'll be surprised how many different muscles speak up especially on the single leg section. Do another two legged session and then unclip the other leg. start with moderate easily resistance and then as your stroke starts to become predictably round, drop the resistance and lengthen the intervals. If the resistance is too low, you will rock.