Try this:
With a Stant coolant pressure tester, test cooling system pressure.
attaches at radiator, you pump up pressure by hand (16lbs?) and watch the needle, it shouldnt move. If it moves you have a leak.
With the same tester , test the radiator cap see if it holds to the advertised lbs.
allot of coolant problems, believe it or not are as simple as your radiator cap. If your system wont hold pressure, it wont work right.
I was told to run the highest rated radiator cap that your system will hold (radiator, hoses, heater core etc), You actually raise the boiling point with a higher rated cap.
And unless you need
antifreeze, straight water with a rust inhibiter is the best cooling agent.
Call the guys at Stewart components (cooling experts) or you can find them online too.
good luck