It would be best if you were more specific with the yearof your model. I have seen several Gen III modelsfail due to thea bent blend door cable. To check, have someone operate the temperature **** while you inspect the valve at the heater core. More than likely , the cable that actuates the valve is bending rather than switching fromcold to hot position or from hot to cold. If the cable IS bending, you can try disconnecting it and straightening it - but likely it will just happen again leaving the only remedy as replacement. If it's not bending, see if there is any indications that it has bent or is intermittently bending.

J

J