The circumstances make a difference - whether it gets hot when idling but cools down when you go, or heats up when you drive and just doesn't cool down, and if the heat has stopped working. My guess is it gets hot and doesn't want to cool down but the heat works. If so, that points to radiator blockage. If the radiator has had tap water added (usually to dilute antifreeze) it is very likely because it takes only a gallon or two of tap water to thoroughly plug up a modern radiator and no flush in the world will help. The test for that is to remove the hoses from the radiator and try to run water from a garden hose through the radiator. It should take all the hose can put out without backing up. The last radiator I changed out, in a friend's 2000 Focus, was so plugged with hard water deposits I could hardly blow air through it.