I have a 2004 5.3l gmc sl modle pickup turck. it currently has a heat problem. after start up the engine warms as per-normal, cab stays cold. the engine temp then exceeds 100 to 120 degrees. If then i give it throttle to 4000 or so rpm. for a short moment the heat returns to the cab and engine temp returns to normal. please help troubleshoot this problem.