2000 Sundeck 5.0 efi water temp sender

[boater2018]

The other day, I started the 2000 Sundeck 5.0 efi for the first time and all was good. Last year, the water temp occasionally jumped some on the gauge, so since I had just had the lower unit serviced and replaced the gauge a couple of years ago, I figure either the sender or the thermostat might be the culprit. The other day, I changed the fuel water separator, oil, oil filter, added the inline check valve to hopefully help with the vapor locking issues we had last year, thermostat, and the water temp sender. Today, I started the boat, the alarm went off at first and the boat went dead a couple of times. I think this may have been because I didn’t fill the fuel water separator first. I did prime it by turning the key several times before cranking, but it acted like it did when it vapor locked last year. After starting it 3 times, the alarm went away and ran as normal. Once it ran for a minute, the temp gauge started going up, but all of a sudden, before it hit 130, it dropped down to the peg (0) and never moved again. When I replaced the sender, I used the Sierra brand and put some permatex thread sealant on it. Would the sealant have caused the ground to be interrupted and cause it not to work? Or are the new senders known to be faulty?