Temp. fluctuations every minute or so

My 98 Kanga's temp guage will go from about 140 to 170 every minute or so. I have replaced the thermostat twice, and my impellor is new and in perfect condition. What should I do? Is this OK since it never overheats? It has been doing it ever since I bought the boat.(all summer) Unfortunately I bought it used and there was not even a thermostat in it when I bought it. Maybe the previous owner screwed it up or something. I wish I knew all the things about boats before I bought my first boat. Next time I will be a wiser boat buyer.

Sounds electrical, ie, the block sensor, the gauge itself, or a ground connection in the circuit. Am not an electrical guy... I would probably jiggle and switch-out parts til I fixed it.

Our 99 outback, 350 Indmar, runs at 174 or 175 steady all day long...