Does the temperature sending unit have teflon tape on the threads, thus not making a connection to ground? Take a look at it. Is the clip on ground strap at the tank in place between the sending unit and fuel line?
I used the old temp sending unit and a heat gun to confirm the temp and fuel gagues and the cluster itself isn't the problem. I could keep looking forever, or I could accept it's not in the cluster and put it back in. So I'm in the process of reinstalling it.