I have the same problem with some of my rocket going wobbly with SAS on, until they reach a height where they are out of most of the atmosphere. I've read that it can be caused by having a bendy rocket in the first place (MOAR STRUTS!!) and to many sources of attitude control, eg fins, reaction wheels and engines that gimble. You tap, d to lean your rocket east, the rocket starts to bend, and all those sources of attitude control start to conflict with each other, leading to more wobble which exaggerates the problem. If the rocket is still controllable without sas turned on, i just run with it anyway, Jebb likes a bit of wobble on his rockets