I have the same problem with Andiamo. There are two choices - you need to learn to concentrate on steering manually - only very small movements of the tiller and watch your course very carefully - or use the autohelm! In fact, watching the autohelm steer the boat under power will give you the clue on how it's done. I remember being very worried by it when I first bought the boat since my previous one (a Sunray 21) could be left to steer herself. You do get used to it!
I have found that a dirty bottom makes it worse.
Having the rudder drop-blade down helps.
Neil
Neil Sinclair
Seal 28/27
'Andiamo of Exe'