Converting a R12 to R134a is definitely a mistake. I don't think the Midas guys can fully understand the AC operation in your MB. As Steve said you need to find a mechanic who really knows MB AC system.
Since you can get cool air when ambient temperature is down. I am suspecting the recirculation flap get failed thus the outside hot air get mixed with the cool air.
I am not familiar with 85 380SE. I'd recommed you take some time to search this site for a clue.
Sorry for not being help.
Charles from Taiwan
94 C220 AMG style