Nissan Repair: Air Conditioning, center vent, nissan frontier

Question
I have a 1999 Nissan Frontier 4x4 with a v-6 engine automatic transmission.  The air conditioner was blowing 40deg air in the center vent. However, as the days got hotter the air conditioner would stop blowing cold air and start blowing hot air.  The center vent temp would rise to approximately the outside temp, in Florida now about 85deg.  I could turn the air off and wait a few minutes, turn it back on and it would cool down to about 40 deg in the center vent for awhile and then start blowing hot air all over again.  I have to repeat the on and off proces ever few minutes.
I was told it was probably the expansion valve so I replaced it and after pulling a vcumm and charging the system again it now only cools to about 50 in the center vent only to repeat the previous process and go up again to about 80 to 85 deg.  Same thing as before changing the expansion valve.  I notice early in the morning when it is cooler outside it does not start blowing hot air as often as it does later in the day after it gets hot outside.
I stopped on the road today when it started blowing hot air, raised the hood and the clutch was not engaged on the compressor.  I turned the air off for about 3 minutes, turned it back on and the clutch engaged and it blew colder air for awhile only to repaet the process of cold then hot.
Please, any help will be appreciated.

Answer
Hi Billy -

You may have some restriction in your system. Try this test - with the A/C on max, spray some water on the condenser fins for about a minute. Check the A/C temp. If it is much cooler, then I would check the in line filter screen (this should be near the accumulator), and ensure the condenser fins are clean. You will have to discharge the system again to replace the screen.

Before doing this, though you might want to put the A/C system on gauges and check the low and high side against normal parameters. This will tell you if the system is clogged or not. If the head side does get too high, it automatically shuts off the compressor to prevent damage to it. Just FYI.

Hope this helps -
aloha
calvin