May have seized DCCV (heater valve) or blocked matrix

 

I have a 2000 Jaguar S-Type 4.0 (No Navigation) and when I turn on the Air Conditioning it blows cold for about an hour of driving and then it starts to blow out extremely hot air....When I cut the car off for a few minutes it start to blow out cold air for awhile and then it start blowing out extremely hot air again when the Air Conditioning is turned on....I took it to the shop to get it fix for $700 and when I left the shop yesterday it starting to do the same exact thing...The shop I took it to obviously don't know what they are doing...Do anybody have any tips??? Thanks