Has anything been done to your system? Did it just slowly start getting worse or just doesn't seem to work like it did last year?
The air gap could be a problem but usually you can hear what sounds like a "squeak" or metal slipping against metal(sorry cant explain it). The air gap on the compressor is the space between the clutch face and the pulley face. This gap must be large enough so that their is no contact between the two. When the coil is energized it pulls the clutch in against the pulley engaging the compressor to pump/compress refrigerant. After high mileage and many, many cycles the air gap does increase from wear. This is rarely the issue however it can happen. Normally you can watch the compressor once it is engaged to see if the clutch appears to be slipping. If this is an issue it will occur when the head pressure is highest, that is at idle with the a.c on max and fan on high speed.
Make sure the blend door is completely actuating, isolating the heater and evaporator core from each other when in AC mode. It is located behind the glove box.
The orifice may be clogged from the desiccant. It is located between the discharge tube and the evaporator core. If you have gauges you will see a low reading on the suction and a high reading on the discharge side. With an orifice system, a restriction will directly affect the pressures being seen where a txv is more difficult to diagnose as the metering will change depending on the temperature sensed by the sensing bulb.
Is the accumulator discharge tube frosting(this is where the orifice tube is located) prior to going into the cab/evaporator? This can help try to diagnose if it is causing the problem. It is difficult as this is a "factory" restriction/metering device.
The suction line to the AC compressor can frost due to humidity and the low temperature of the refrigerant so I am not talking about that,(trying to help locate a possible restriction for those thinking this).
Are there excessive bugs or debris in the condenser? Bent fins?
There has to be enough air moved through the condenser to properly allow refrigerant to properly change state(vapor to liquid). Any blockage or reduced airflow can cause your AC system to work improperly.
If you have a large fan that can move ALOT of air, try putting it directly in front of the condenser(simulate at least 35mph). Using a thermometer see if there is a difference in discharge temp with and without the additional airflow.
