Aero foil rudder speed limitation

[Ian W]

Hello,
My recently acquired planning TSMY is fitted as original with GRP aero foil section rudders.
All but my vessel were fitted with twin 210 hp diesels to exposed shafts supported by
'P' brackets.
However, my boat was custom built in 1976 and fitted with 300 hp diesels and using the same rudder pattern as the 210's.
The maximum respective speeds are approximately 21 and 28 knts.
Before I become too paranoid about the strange steering characteristic I am experiencing, would anyone be able to throw light on possible causes?
To this end are there limitations with any state of the boats trim or speed when using this design of rudder as opposed to a more flat section bronze rudder used on similar boats?
Thank you in anticipation.
Ian.