R134 Conversion Woes - Evaporator Freeze-Up

Converted the RV2 on my '74 Duster to R134 when I swapped the /6 for a 340 a few months ago. The air worked great prior to the swap. Had new hoses made to allow for the repositioning of the compressor. Vacuumed the system. Changed o-rings, valve cores, etc to convert. Installed R134 & ester oil at 80% of R12 capacity. Blows cold (not R12 cold, but cold enough) but twice now when I've had the thing out on longer runs (picked the wife up at the airport in it last weekend - about 70 miles roundtrip) the evaporator has frozen up. I've heard of this happening, but never heard what the fix is. Any suggestions? Should I replace the receiver drier to elimate the possibility of trapped R12 particles, and could this be (part of) the cause? I'm sure the Florida humidity is not helping me one iota either... Any help will be appreciated.

Ray