How can i fix my cars cooling system?

My 2000 Tracker (GMC) started overheating. I noticed a small leak in the rad and put in some Barrs stop leak. The overheating continued and I went to a garage. They replaced the radiator, a six hundred dollar charge. The problem did not go away. They have now replaced the clutch on the fan, the thermostat, checked the hoses ect. at a cost of another 500 dollars but still it overheats.
They bipast the heater and the vehicle runs correctly. They advise me that the Barrs is likely blocking the heater core for the cab and have told me it is a six hour job to replace. Can you give me any advise on how to correct this problem at less expense? Thank you