Steering Problem on Cobra Equipped Boat

[blueoval1]

I have a 1987 17ft bowrider that has a 4.3l Cobra setup w/power steering in it. When @ no wake speed (as in idling or trolling around) the boat is a constant struggle to keep it straight. I have to continually move the steering wheel back and forth to keep the boat on course.
Also when turning the steering wheel , the drive shakes a little at this speed.
Can anyone give me any pointers as to what needs to be done to solve this?
It drives me crazy when I go to the dock!

Thanks for any help!