Could be the pushbutton control unit.
My 87 560SL did that....it would be great and then, all of the sudden, part of it would quit. Meaning the blower would stop (but the AC would continue). The CCUs are really 3 components soldered together and with time and usage, those solder connections break and gremlins get into the workings.
You can get a rebuilt one for about $150 from an outfit in Georgia.