autopilot random wandering

[Bhngh]

I have an S2G course computer with a ST600l helm controller.
It has been working fine for several years.
The rudder sensor unit was replaced a couple of years ago.
The last 2 trips it has worked for most of the day traveling at
least 100 miles on each trip. That is until toward the end of each trip.
At that point, it starts randomly changing course. 15 -20 degree changes.
It isn't sending any error messages to the helm controller.
It seems to work fine on the trailer. I set a course with the mfd, then lock it
in to "track" as I've always done. The autopilot does what it's always done at that point. That's on the trailer.
What should I check before I put it in the water and attempt to trouble shoot this
seemingly intermittent problem?