United Airlines 777-200??????

I got a quick question. I flew the Posky UAL 777-200 yesterday and when i turned on my nav switch it started rocking side to side. I tried turning the nav off and putting heading hold on but it didn't work. so i turned heading off and put on wing leveler but it kept doing the same thing, so i turned off autopilot all together and it stopped. What is this and how can i stop it from rocking liket his in flight with AP on?? Thanks for ne help!!!