What causes an AC evaporator to freeze up on a car when the AC is cooling great on normal cool?

Dirty condenser coils or low of refrigerant. I need to elaborate a little more. System is not low of refrigerant. 2000 RPM high side pressure is around 310 psi low side pressure is around 17 psi. A/C cools great, but compressor never disengauges and evaporator freezes up. Condenser is clean on the outside. I'm thinking that something may be stopped up. Expansion valve maybe? Or is it just a switch? I doubt that it's because of low refrigerant! I'd check the cycling switch first...