Temp Gauge Help

I am having a weird problem with my temperature gauge. Initially, it worked some times. After I rewired with the painless kit, It pegged quickly. So I disconnected the sending unit wire and replaced the sending unit. With the new unit it reacts weird. If I run the engine it will peg, however if I disconnect the sending unit and let the motor warm, then shut the motor off I can re-hook the sending unit up and just turn the ignition to the on/run position and it shows the approximate temp. If I start the engine up the gauge will automatically peg. Any ideas????? Just a little info I am not using the stock fuel gauge since the PO replaced the tank with a 25gallon tank and put a different sending unit and gauge on, however I run the +12v in the correct place on the fuel gauge to ensure it goes through the regulator.

Thanks in Advance for any help, this board is an extremely valuable resource....

 

So it works ok as long as the engine is not running?

 

Yes.... As long as the engine is not running, it works fine.... I am working back checking for shorts, but wasn't sure if anyone had seen anything like this before.

 

Have you made sure the 5V regulated source is at the proper voltage when the engine is running and when off?

 

I am working on that now, I have to pull the speedometer assembly to get a multimeter on it. I am going to pull it on Friday.