Are they actually going bad? They produce a variable frequency centered somewhere around 4kHz (I'm guessing here) and varying either side of that by something like 20Hz (again guessing) for each degree of movement.
When you send them in, does Simrad scope
them and tell you that they are indeed bad?
feedback error can also result from the clutch
going bad on the drive unit, or the power not good to it. It can also be internal to the course computer itself.
If the rudder
unit is indeed the problem, or if the problem is a bad circuit in the course computer and you have an N2K system, you could get the RF25 and connect it straight to the bus instead of to the course computer.