R134 Conversion Woes - Evaporator Freeze-Up

My fiance' and I were just having this exact discussion this weekend. I had mentioned that I had converted to R134a and that I should have evacuated the system first as the vent temperature rises when I'm at idle. She asked me if I had replaced the dryer. I said no and she said to be prepared for a freeze-up due to left over R-12. BTW, she's a BMW master mechanic. I can ask her for details tonight, but I'd bet it's your dryer.