On my "put away" trip to the boat yard, my ST6000 suddenly gave me a unusual message...something about the course exceeding rudder stops, or the seas were too heavy to hold course. We were playing with the Radar/Chart at the time so I chalked it up to something we did. I shut everything down and fired it back up, but we continued to get the same message (I can't remember the exact error message). I tried it without the Radar/chart, but it just wouldn't hold course for more then a few minutes. We were motoring at the time and the seas were calm, etc. Nothing that would have caused stress on the rudder and/or drive. Any clues as to what might have happened or how to correct the problem.