GMC Repair: Intermitent heat, windshield defroster, flow control valve

Question
  I have a 1997 GMC Jimmy (S10).  My problem is the heat from the vents and windshield defroster will come out cold intermitently. The cooling system has been flushed out multiple times.  The radiator has been recorred.  I'm starting to think about the heater flow control valve. On this particular car Where is it and how to replace it...  

                             THANK YOU

Answer
First I need to know what is the past repair history of the vehicle and what was the last repair of any kind what-so-ever was performed on the vehicle prior to this problem ?.

Why did you recore the radiator when a NEW radiator only costs a few dollars more. ?.


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