ruder vibration

We have just bought our First 31.7. It is only 3 years old but has a significant vibration/movement inthe rudder/tiller under power. The tiller is quite stiff under power, bu under sail it is fine, well, superb actually, very light and precise. Have lifted it out of the water and there is some play in the lower bearing, though not as much as the vibration would have suggested. Also, prop looks fine. Has anybody else had any similar experiance with rudder bearings on the FIrst 31.7 (or any spade rudder for that matter)?