Exact same problem while offshore
State, Strait of Juan De fuca. boat turned east 90 degrees and started heading for shore without any indication of a problem. Shortly after the autopilot
drive completely stopped working. I traced problem down to a failure of the + and/or - motor
drive signal at the back of the St4000 unit. I think these units may be getting into a condition when the positive or Negative power drivers start failing, otherwise the power drive transistor intermittantly starts swinging between +12 and ground, even though its getting the correct bias input signal. However this is just a theory at this point.