water temp.gauge works erradic till engine warms up
Coolant might be low, or could be air in the cooling system. Do you hear any sort of gurgling sounds when engine is running, maybe worse when turning on heater?
Most likely it is a problem with the soldier in the circuit board. sometimes they get little cracks in the soldier. things expand in the heat. when cold the crack opens up and does not make a good connection. when it heats up it expands and makes the connection tighter. I had this happen with a gas gauge in the winter. once the cabin heat was on for a while it would work fine.