Could be an overcharge so that the high pressure switch is shutting down
the compressor to prevent a blowout. Check the high and low side
pressures and see if they are consistent with the ambient air
temperature (tables are provided in most service manuals).

Could also be an inoperative cooling fan allowing the condnsor pressure
to get too high. Make sure the cooling fans are operating when the AC is
on, and switching to high speed (or kicking in an extra fan, however
that system works) at high temps.

    How did you check the freon charge?  By eyeball?  ("...showing
OK").  The only way to be sure is with a good set of gauges, measuring
system pressure.
    The amount of refrigerant might be a little bit low.  That can
cause the suction pressure to go low enough to shut off the compressor
clutch.  There's a pressure switch to cut power to the clutch in the
event that refrigerant is lost.

  Sounds like your condenser core is probably dirty.  Hose your
condenser down with parts washer, and GENTLY rinse it out.  A direct
blast from a pressure washer can lay the fins over and block air flow.  
In fact, maybe someone already did that and it's what's causing your
problem.  If the fins are all folded over you might be able to get a
"fin comb" at a parts store and straighten them back out.
  Also check for leaves and crap in between the core and the radiator
or a failed fan.  It'll cause the same problem.
  A dirty/blocked core will cause higher head pressures on the high
side of the system, and if it gets too high the compressor will cut out.  
If that goes on too long it'll char the oil and cause mechanical
problems.
  If that doesn't help, you'll need some pressure gauges or at least an
IR thermometer go do much more troubleshooting.