The heater on my 2003 Chevy tahoe 4x4 5.3 sometimes works and others blows cold air. My anti freeze level is fine my cap is tight. They told my thermostat is fine

 

Welcome from Florida. Check to see if both heater hoses are equally hot. If they are not you probably have a clog in the heater core. If they are both hot then it's probably the blend door actuator.

 

Welcome to the forum. Moving your post to the Tahoe section where you will get more views.

 

If the two heater hoses are hot 10 minutes into driving, then the issue is under the dash. Since you sometimes get heat and sometimes not, I'd suspect a faulty "door" or flap in the air flow system, or in one of the actuators that move those.

 

My guess, along with "oldchevy" is your heater core is plugged. That is a common GM problem, but with an easy solution. Your car is 20 years old. Disconnect the heater hoses and flush it out both ways.

Tom