I finally found the problem. The four power transistors which drive the motor
are mounted on heat sinks on the circuit board,. The problem is that they have short, straight thick leads which have no bends before going thru the circuit board. Due to normal heating
the expansion slowly cracks the solder connections. Even though the cracks are so miniscule they're almost invisible, it's enough to cause intermittent operation. I soldered up all of them after looking under high magnification. Out of 12 connections (3 on each of 4 transistors) there were NINE bad connections!
We just got back from a 3 day sail and what a difference! It's just like new, and doesn't correct nearly as much as before, but it keeps a PERFECT course.
If anyone's interested, one of these days I'll take it apart again and post pictures of the offending spots.