My jimmy did that 2 years ago. Its a 2000 and one shop told me I needed to replace the Heater core. I took it in to a shop that works on the radiator and told me it was clogged up with radiator fluid. They did a huge flush. They said the made a special tool. They seen so many jimmy's with this problem and they fixed it. A huge flush cost me 300 and worth it. A new heater core is almost 1000. Try a deep flush. Maybe a radiator expert shop.