Thanks for the reply, Benzmac.
I just had my coolant drained and refilled, and it's nice and full.
The aux fan (the one in front of the radiator, right?) doesn't kick on in unison with the A/C being turned on, but does turn on shortly thereafter (when the temp gauge gets above 80).
I guess it must be the weather that's affecting it, but here's my remaining concern: What will 110 degree F desert temperatures do to it?? I don't want to get stranded in the middle of Nevada, and would like to keep my A/C on throughout the trip. I have to imagine that the car was engineered to withstand such temperatures for a couple hours, but I just want to be sure that I'm in spec during normal conditions, so that when I push it to the limit, my temp gauge stays safely away from that 120 degree red mark!