I have a 1999 Land Rover Discovery 1 SD and having over heating issues. I can drive it all around the neighborhood and it doesn't over heat. If I drive it 40MPH + for 1 mile or 2 it will pig the needle. I installed a new thermostat but no luck. Water is circulating through the radiator. The clutch fan is operating, 1 out of the 2 small fans that are mounted on the front of the radiator is not running. Could that be my problem?