Ok i have a JG pro series block with closed decked in my car. I have all new sensors for the thermostat and a new thermostat too. I also am using a civic Fluidyne radiator. Now the problem i'm having is that the stock tempature gage sometimes goes way above where it should be then falls sometimes very quickly but sometimes comes down very slowly. I do know that the fan is kicking on when the motor starts to get hot and it stays on until it cools it down. I'm only running about 5psi and have about 1200 miles on my new rebuild. Any help would be greatly appreciated.

 

could just be very bad tuning, massive ignition retards will do that also.
check that water pump functions normal and that thermostat opens properly and stays open, get a real water temp gauge to know for sure when you hit to high.

 

Thanks for the help. I'm going to get a good water temp gauge to monitor my motor.