I've got a 1999 Nissan Altima and the Air Conditioning doesn't seem to work
any more.  Hopefully someone can give me some insight into my problem...

When I discovered that it wasn't working very well, I assumed that it was
low on refrigerant.  So I went out in the morning when it was cool and
released a bit of refrigerant out of the high pressure side to see if there
was anything left in there.  There was definitely still quite a bit of
pressure there.  I decided to "top it up" with some R12A refrigerant
anyways.  I've done this a couple times before and it always seemed to do
the trick.
While I was topping it up, the air coming out of the vents was good and
cold.  Later that day, I had it on the highway in the sweltering heat and
nothing comes out of the vents but warm air.  I thought I could feel the
compressor kick in every minute or so for about 2 seconds (loaded the engine
down a little).  And I thought I could even feel a little blast of cold air.
I'm not sure now whether that was the compressor or the electric radiator
fans kicking in.

I've checked the two fuses and even swapped the A/C relay with another one
to see if that was the problem.  Sometimes when I start the car up in the
driveway, the radiator fans turn on right away as long as the A/C button is
pushed.  Then they go off after about 30 seconds.  It doesn't seem like the
compressor clutch is even trying to engage.  If I hold on to the A/C relay
and get my wife to click the A/C button on and off, I would think I should
feel it click but it never does.

Does anyone know of something else I can try?

Thanks for any help
-Trevor Heppler