The air-conditioning system cools satisfactorily during the early morning or late evening, but does not cool during the hot part of the day.
The problem is that the A/C system seems to work fine in the early morning and evenings, but does not during the heat of the day. During a performance test, the low-side gauge reads normal and then drops dramatically.
Ice could be forming on the expansion valve
The receiver drier could be saturated with moisture
Moisture in the system can be released from the desiccant in the receiver-drier by an increase in temperature on the high side. When the ambient temperature is lower the desiccant may be able to hold the moisture out of the refrigerant. Internal freezing of the expansion valve could produce external icing on the valve.