Have you done any work to the rad, hoses, etc. lately? If not, you may have a leak in the system somewhere that needs to be fixed. Assuming hoses are on correctly and you have no leaks, you can try this: wait until engine is cold, take rad cap off and run engine until it warms up. Should see air bubbles coming out, when that stops THEORETICALLY your problem is gone. BE CAREFUL, once engine really hot, fluid will boil out! Don't wait that long before putting the cap back on. Good luck.