1995 BUICK Park Ave air conditioning intermittant hot and cold

Sometimes the A/C comes on and works perfectly, other times it comes on HOT and takes several seconds befroe it sorts itself out and blows cold. I’m assuming it’s a flapper door issue, but what is the control for those flapper doors? Is it the ECM or is the control for the A/C solely in the gizmo (Is that a good automotive term?) where I punch in the desired temperature? It has one of those set it and forget it thermostats. It should stay on 75 all the time, but doesn’t.