ACU200 Eventually Starts Steering Wildly

I have an ACU200 Autopilot, ST4000-II wheel drive and a P70 control head. When I first engage the autopilot, it works beautifully -- a small red bar, the boat turns slightly, then a small green bar, it turns back. And on and on...

After about 45 seconds, the turns get wider and wider. Hard to port, hard to starboard. No green bar on the display will show. In no time, the turns get so wide, the alarm goes off and goes back in to standby.

I shut it down, reset and it works great again for about 45 seconds, then the S-turns get wider and wider again.

Any ideas?