Mine does the same
My 25 is a 1981 model and it does move maybe 1/2 inch up or down. Has never been a problem in the 5 years I have owned the boat. I believe on mine you can push it down but it normal is in the up position meaning the rudder is lighter than the water it is in. If rudder became water saturated in might settle in the down position. The collar which I replaced could be moved to take out all the slack. This would require redrilling the shaft hole the collar is attached or bolted on as an assembly. I replaced my collar when I purchase the boat as it was cracked and only half was left.