Temp Gauge sometimes goes all the way to cold

@jenniferto23705 since you have the proper amount of coolant and your truck is not overheating, it sounds like a bad coolant temperature sensor. Let us know the engine size so we can try and find the proper part, troubleshooting procedure and location for the sensor.

Of course, it can always be the gauge itself. My instrument cluster (2003Ford F-150 250k mile) is a bit flaky at times as well. But at least we should try and narrow it down for you.

Just in case it's the gauge, here is how to check it.

TEMPERATURE GAUGE INACCURATE
1. Disconnect Sensor - Ensure temperature sensor connector is in good condition. Turn ignition off. Disconnect coolant temperature sensor. Turn ignition off. If temperature gauge indicates cold, go to next step. If temperature gauge does not indicate cold, go to step 3).
2. Bypass Temperature Sensor - Connect a jumper wire between the 2 pins of temperature sensor harness connector. If temperature gauge indicates hot, replace coolant temperature sensor. If temperature gauge does not indicate hot, go to next step.
3. Check Red/White Wire For Short To Ground - Turn ignition off. Disconnect instrument cluster harness connector C237. Measure resistance between instrument cluster harness connector C237 pin No. 9 (Red/White wire) and ground. If resistance is greater than 10 k/ohms, repair/replace instrument cluster. If resistance is less than 10 k/ohms, repair short to ground in Red/White wire. Check gauge operation.
4. Check Red/White Wire & Pink/Orange Wire For Open - Turn ignition off. Disconnect instrument cluster harness connector C237. Measure resistance between C237 pin No. 9 (Red/White wire) and pin No. 5 (Pink/Orange wire). If resistance is less than 5 ohms, repair/replace instrument cluster. If resistance is greater than 5 ohms, repair open Red/White or Pink/Orange wire. Agregar imágenes a una pregunta existente

Sounds really complicated and all but it's more tedious than complicated. You can always elaborate if stuck or post images of things that are odd and you need more information about it.

You know You got This!