Tacktick intermittent knotmeter malfunction

[beepbeep]

After years of working properly my tacktick knotmeter is getting quirky. One day after working well, it abruptly started reading much lower speeds (say 0.6 knots down from 6. That reading then decayed to zero over a period of perhaps 10 minutes. Back at the mooring I pulled the triducer and spun the paddlewheel. It spun freely and gave a very low reading for a while and then went completely to zero. Depth and water temperature red correctly. The next time I used the boat everything was fine. Then several days of failure. Once or twice I could get a very low temporary reading by turning the instruments off for a few seconds and then back on. Finally yesterday it started out working properly for about 20 minutes and then decayed slowly from 6.3 knots down to zero over a period of about 10 minutes.

What should I test? Which replacement part should I consider?