Why would the evaporator coil freeze up every evening shortly after dark when it is fine during the daylight hours?

It may be over-charged (too much refrigerant) or not enough air across the evaporator coil. Actually it could be under charged. The pressure is lower during the night since it is cooler outside. If the pressure drops on the low side below 56-58 pounds correlates with lower than 32 degrees making the humidity in the air freeze to the coil (evaporator). In all likelyhood you need a slight charge of R22. If the filter is dirty, again it could be like the 1st answer not enough air over the coil. But I will stick with my theory.