we are having difficulty getting continuous cold air from the dash A/C. It starts cool and gets very warm quickly, 110 plus degrees. we are wondering if the control box for AC(slider) is not working and has anyone else had this problem? We are wondering if we should bypass the shut off valve.

Based on the system in my old Regal there are several things I would check:
The AC and the heater use the same common plenum. The temp control on dash was a cable that runs to a water valve in the heater hoses. Be sure the cable is closing that valve when AC is used or the AC will have to fight the heat.

The remaining heater controls were used to select plenum flap controls including fresh/recirculate control. Make sure your system has working vacuum source.

Be sure your AC system has a full charge of refrigerant. Partial charge may cause intermittent operation.

Good luck.