Water Temp Gauge Acting Up Options

[HardHead93]

I have a 5sfte with a DIYPNP standalone ECU. My water temperature gauge started acting up recently. When I turn the key to the on position (car not started and cold) the needle goes all the way to hot. Then when I start the car it drops all the way to cold and stays there. When I get the water temp reading off my ECU it shows the temperature as normal when the car is fully warmed up so this looks to be some type of problem with the water temp sender, wire, or gauge. I have replaced the water temp sender with a brand new one and the problem is still the same. I have looked at the factory wiring diagrams and it looks like the water temp sender (different from the water temperature sensor) is wired directly to the gauge (no ECU connections) and is grounded at the intake manifold. I do not feel comfortable driving the car with no insight into how the car's temperature is performing and I do not want to have to have a laptop plugged into the ECU all the time to get that information. What would be the best way to test the wire and gauge to determine the problem? Has anyone experienced this problem?