RE: Reason for not upgrading to 2.4
I checked for the firmware update. The Rx comes back on with solid lights (no flashing) about 1-2 secs after cycling power. It sounds like what I experienced was indeed common under specific conditions. My system is headed back to Horizon for the update.
onewasp - your guess seems right on. I tested voltage while cycling servos, although I'm sure the airload on the surfaces puts way more load than what I can get on the ground.
havinfun - I keep my Tx antenna at the 45. Since I don't have the updated firmware, I'm leaning toward thinking that was the most likely cause.
One thing I noticed when going from 72 to 2.4 was how much more precise the control feel was. I had to adjust my rates/expos after the first flight with Spektrum. What I probably never noticed was how much voltage drop I was getting with my old set up since 72 tolerates it.
onewasp - Thanks for the mention of A123s. It confirms my decision to go with a better battery set up.
Final resolution - Upgrade batteries - Update firmware.
Thanks to all for the spirited and educational discussion!!