Thanks to all who sent replies and information.  I found today that it is
not a problem with the refrigeration itself.  After driving for 15 minutes
with the A/C short cycling it decided to stay on, and output temperatures
dropped to 45 degrees for the rest of the drive home - this at an outside
temperature of 100-102 degrees.  This seems pretty good to me.  The problem
is the control mechanism.  I've checked the relay cold and it doesn't look
burned or anything else melted - although it was replaced last year about
this time because of this same problem, and the old one did look like it
had overheated.  I'll have to pull off the panel and check the relay while
the A/C is running.  I checked the drain line where it comes out in the
right rear quarter panel, and it blew clear, but I haven't yet gone into
the A/C housing. Turning the A/C on and off or turning the blower up to 4
was all I tried to get it to stay cooling.  Other times it has started
staying on all by itself after a while.