CX7 forum provides the answer (again)
This is such a great information resource here...and I just benefited from the knowledge again.
Our '07 GT ran great on the way to/from Cedar Point (major amusement park in Ohio on Lake Erie, if that doesn't ring a bell. Speed-needing folks MUST try the Top Thrill Dragster ride, 120mph in 4 seconds, then 420 feet straight up...and straight down). About 500-ish miles from St. Louis, four of us and luggage, 23-24 mpg.
But then.
Coming back yesterday, upper 90's, running 70+, in western Indiana on I-70, the a/c stops being cold. I tried off/on, different modes, etc. which were no help. Stopped for gas in Effingham, and my mind was thinking back to aftermarket a/c units "back in the day" that would "freeze up" under severe use. So it didn't surprise me when the a/c began working once we got underway again...and that it stopped again 8 minutes later.
So it was a long, hot, final couple hours into St. Louis. We stopped for dinner, and then it worked fine just after as we drove home.
So I dialed up the forum today for a little research before I called the dealer after they opened, and found this a/c TSB...which describes the problem perfectly. I had visions of rounds of diagnostics, "it worked fine while we had it", having to take field service techs on 4 hour road trips, whatever, to reproduce the problem. But not now. Is a known issue with a TSB (long distance, 100'sh temps, condensor anti-frost device doesn't work properly).
I have not had the "no a/c at stops" issue; of course it produces less cooling with lower engine speeds, going to recirc helps some, but it never stops entirely.
Speaking of TSB's, I did get the engine (idle hesitation / power smoothness) and transmission ("surge" or pulsing at highway speeds) applied at my 7500-mile service and they perform as advertised. The power application from a standing stop is much more well-behaved, and the surging at highway speeds is gone. Only thing I did notice is that now it doesn't want to give me the "best" gear on downshifts...it won't give me 3rd from 6th when I think it should and that it used to. No biggie.
My dealer here had not heard of the door seals TSB, so had to order a set for me. To me, it's a shame Mazda isn't doing a recall on those; I'm envisioning lots of unhappy CX-7 owners, esp. over the winter, bitching about how bad dirt/salt/grime build up on the inside of their doors, "seems like Mazda would design it better", "my Toyota never did this", when the fix is known and available. Sigh.
Anyway...too much rambling, but just thankful to find this a/c TSB so quickly!