my truck seems to be running cooler and cooler. the gauge used to be set pretty much dead center when it warmed up. now it barley comes of the cold. i dont know if it means anything but i have a portable temp gauge and i put it to the radiator hose after about and hour of highway driving and my top hose was only at 117 and the bottom hose was only at 59 degrees Fahrenheit. Any ideas what might be wrong?

Thanks