2001 Bravada Heater/Engine Problem

I have 2001 Bravada. After running awhile the heater turns cold and the engine temperature gauge starts climbing. I have replaced the themostat but, the problem continues. And, it doesn't happen all the time. Any sugguestions?

Sounds like a low coolant level in the radiator. With the engine cold, take the radiator cap off and check the coolant level. Add coolant if it isn't full to the neck.