That R134a and R-12 have different cooling efficiencies. If you converted to R-134a, then your cooling efficiency will be lower than before, when you used R-12. The only way to get better cooling, I believe, is to install a complete R-134a system, including new compressor. It's probably a higher capacity system that makes up for the ozone-friendly refrigerant's poorer efficiency.
If your cooling woes started well after converting - I'd have some dye put into the system before assuming that the evaporator is bad. You don't want to replace an evaporator for $3000 only to find out that your problem lies somewhere else.