Chevrolet Repair: 1997 Chevrolet Pick-up A/C, vacuum line, mechanical work

Question
I have a 1997 Chevrolet pick-up (c 1500 series) with an a/c problem.  When I turn on the interior controls the a/c will not come on.  As I drive the cold air will begin to blow through the vents but it will stop circulating when the vehicle stops.  This also happens with the heater as well. On occation the unit will begin to circulate the air if I turn it on the very first time when I crank the vehilce in the morning but will not contine for very long.  When the air is blowing it is cold or as hot as it should be. It just will not circulate unless the vehicle is moving.

Answer
Explain in more detail as to: "When I turn on the interior controls the a/c will not come on.

WHAT won't COME ON ?.


What mechanical work or ANY kind of work has been performed on the vehicle recently ?. As most problems that occur after a recent repair was made can be traced back to the area of where the repair was made as a vacuum line may have been knocked loose or a wire connection came loose, Look around in the area where work was performed.