My water temperature gauge is erratic. When i first launch the boat, it reads normal. Then after i take off, it drops down to nothing. When I run it on the trailer with a garden hose, it reads normal. I checked the gauge by removing the signal wire that runs to the sending unit, and jumping that terminal to ground, and the needle goes all the way to the right. I am getting around 12V on the wire to the sending unit. I replaced the sending unit yesterday, thinking it was the culprit - no such luck. When i am on the water, and have the engine off - i can turn the key to the run (not start) position and the gauge will read normally. I checked the main ground from the battery to the motor, and it's tight. Any other ideas?
My Boat: 2000 Rinker Captiva 192, 5.0L Mercruiser, Alpha Drive.