I have a recently rebuilt 289 in a '66 Mustang. The 3 core radiator and water pump were new when I installed the motor. The motor ran between 180-190 degrees. After a few months the temp started creeping up. I checked the fluid level and saw rust debris in the radiator clogging the cooling passages. I took the radiator out had it taken apart and cleaned, drained the motor and put new coolant back in. Now six months later I have the same situation. Any suggestions on how to get fix this?