A/P Problem or Joystick Problem?

Hello everybody,I've got a problem with B737-700 NG autopilot...Randomly, during flight (climb, cruise, etc etc) the A/P disconnects and engages CWS mode (p and/or r) and the plane begins pitching down and banking...I know what CWS means but I don't know why it automatically engases by itself...I don't move any axes of my joystick and I disabled it from Fs too, but there's always the problem.I read old post about this topic but all said that the problem was generated by accidental moves to the joystick...It's not my case, because I tried also with Joystick disabled controlling pitch and roll by keyboard.Thanks for your helps...Bye.P.S. Sorry for my English :)Marco