Mike,
You have a different problem that is not related to your TX but your pack.
You need to cycle your NiMH pack 4-5 times with a good cycler to get your capacity back.
Then you can fly down to 8.8 volts with no fear. Here is my example.

I use a 2700maH NiMH pack on my ignition. After about 6 months, I noticed that my ignition would die while flying. I could only get 4 flight before the pack was dead. I would charge it and then go out and fly and only get 4 flights and it was dead again. I knew there was a problem so I cycled the pack. the capacity was something like this:
cycle 1 560maH
cycle 2 800maH
cycle 3 1200maH
cycle 4 1900maH
cycle 5 2300maH

For some reason, the pack would not take a charge until I cycled it.
Now I can fly 15-20 times and the pack is still charged.
What I am saying is if you continue to use a NiMH pack and never fully discharge it, you will loose capacity.
Now, you need to ask Red in the battery forum to find out why? I don't know the chemistry.
I do know that every 6 months, I cycle all of my NiMH packs to ensure that they retain their capacity.
I also NEVER use NiMH packs for the airplane electronics.