Temp gauge bouncing from cold to hot and back to cold over and over again.

What does the fuel gauge do? Are you CERTAIN that the cluster is grounded?

You REALLY need to get either some test resistors or a variable rheostat so you know what you are up against.

You have made a lot of changes, NEVER assume that "new parts" are "good parts." Could be a bad connection in the board, something wrong with the gauge, or with the RTE.