I dont know the motor
type, took the boat from eastbourne to scotland
this week, rough ride and pain in ass hand steering
I had little time to have a proper go at sorting it,it does sound like lack of power problem ie ok when no force on rudder
but it cant power enough to overcome rudder forces on move, so possibly connection dropping voltage or maybe worn brushes/dirt dropping power in the motor, works fine at very low speed as soon as some force is required it quits!
it has a raymarine
st6002 display, cant remember the brain number, will need to take a good look at the brain and motor numbers tomorrow when i take a better look