Blend door issue with A/C in '97 Buick Lesabre



I have had my 97 buick lesabre in the shop for most of the summer now with a blend door issue. You’ll be driving down the road and all is well, a/c working like a champ, and suddenly-no air,not even a breeze, but you can hear the a/c working just nothing coming out of the vents. My mechanic says the blend door is sporadically closing and has tried everything (except changing the cellunoid which he says either works or doesn’t) including changing out the control panel. Help?!