Toyota Repair: A/C charged OK, but will still blow warm, toyota avalon xls, automatic climate control

Question
I have a '96 Toyota Avalon XLS, 3.0L, Auto, 131000 mi. that has the automatic climate control feature.  The A/C was not always blowing cold, it would start cold then would all of a sudden begin blowing warm.  So, I recently had the A/C recharged by a reputable repair shop, and they only had to add 1/2 pound of R134A.

However, this did not solve the problem, as I discovered when I picked up the car.  The refridgerant from the shop had the dye added when they recharged the A/C, and there were no leaks discovered when I took it back a week later to check that out.  So, all seems to be fine there.

The problem is this:  Many times, but not always, the A/C will blow cold immediately from a cold engine start.  By the way, it does not matter if I am using the auto climate control feature or not in these scenarios.  Then, after a while, could be minutes to nearly an hour, the air starts to warm up to the point it feels just like plain vent air coming out.  The A/C light on control panel remains steady green.  I can see that the compressor is running, and can hear it creating a load on the engine.  I don't recall hearing the dampers inside the dash area moving.  The airflow continues flowing through the outlets that were selected, which is usually the upper setting.

It almost acts like there is not enough refridgerant to keep it cold or something, even though it has a full charge.

Once the air begins to warm up, it usually stays warm until I shut the car off at my destination.  It may be cold again when I start the car again after a time, or it might still be warm.  I don't know what would cause a difference there...

What are your thoughts?  Any help would be greatly appreciated.

Thanks!

-Todd

Answer
Could be some sort of blockage in the system possibly an expansion valve or receiver drier unit, pressure readings are a very important tool when diagnosing a/c problems, the first thing I would do if you brought the car to me is to hook up pressure gauges and see what the readings are, if you could provide me with the pressure readings on the low and high side of the system with the compressor engaged I might be able to have some sort of diagnosis as to what the problem may be and what to look for.
Without this basic knowledge I can't be of any more help.