It's not a problem with your design, the fault lies in stock SAS. Any craft told to point pro/retrograde (or other directions determined by orbit) will wobble, as the SAS is overcompensating for its own movement - unless being pointed exactly in the direction you want it to and having no thrust offset torque whatsoever. And even then it's not going to last, because when you thrust, you change the orbital parameters and therefore the precise position of the node itself, which the ship is going to try and follow - and overcorrect itself. You can observe this effect more clearly when you tell your craft to point somewhere and it has to turn around on its own - it's going to go full reaction wheel/RCS power until it reaches the correct position, and ONLY THEN it's going to try correcting any momentum that's left (which is a lot, usually). This causes the craft to wobble back and forth over the chosen node as it overcorrects a little less each time. The reason why simple stability assist doesn't wobble is it's not trying to point anywhere in particular, it's just trying to kill any rotational momentum by producing torque in the opposite direction, until the ship is steady. The solution is simple - use less thrust (so the ship has less momentum to overcorrect), turn on smooth controls (caps) or use stability assist, and pray for SQUAD to implement a better node tracking method.