OK... here goes (I apologize for the length of this post)

1. Last fall I washed the engine on my 1998 3.8L v6 Mustang using the
usual
engine cleaning foam.

When finished I noticed air no longer blew from the ac/heater vents
but only
from the defrost vents. I poked around and discovered a vacuum hose
was
disconnected. I reconnected and worked fine for a month or so.

2. Next thing to happen was the ac/heat thermostat knob became
slightly difficult to turn to the ac side. Turns easily to the heat
side. It feels like some type of spring or cable attached to the knob
is damaged, worn, or out of adjustment.

I haven’t torn the dash open to find out.

3. A few months later I could no longer get cold air from the ac.

Air blows from the vents but never gets cold. Heater works fine.

Took to a mechanic to check refrigerant level. He said refrigerant was
fine.
He jumpered the ac compressor directly forcing it to come on.
He said the problem was the ac compressor was not getting power.

He wanted to charge me to replace entire HVAC system ($900.00+).

I told him I’d think about it (meaning "are you crazy?!")  

4. I did some research and heard about the ac clutch switch (on
condensor) sometimes failing and therefore not allowing ac compressor
to kick in.

5. Took a paper clip and jumpered the ac clutch switch contacts (on
the plug going to the switch on the condensor) and sure enough the ac
compressor seemed to come on (did this with car running).

Went out and bought a new switch and installed.

Now here is what happens...

When I turn on the AC (normal or max) the compressor seems to kick in
only for about 2 seconds then turns off. I hear the engine RPM’s slow
down (and see it on the RPM dash gauge) then the RPM’s go back to what
they were before turning on AC.

A few seconds later the RPM’s go down then back up.

I turn off the AC and the RPM’s stay steady.

As far as I can tell the ac compressor is not on long enough for the
air to get cool (assuming the air would get cool).

Do I simply need to add refrigerant to fix the compressor "quick
cycling"?

Thanks!