I have a H9 33% Cap with digitals all the way through, mostly Hitec 5925's, and had problems with it dropping the RX buss voltage below 3.7 volts and causing the RX to go into throttle failsafe.
I was running 2 NiMH packs, 2 switches, battery backer, optical isolator... Drove me NUTS, finally I pulled all the junk out, went with 2 Li-Ion packs and adjustable regulators and VOILA no more problems, turns out it was 3 things, battery packs just couldnt flow the juice under hard loads, the battery backer consumed .9 volts...
I ended up using a Power Expander, but it is not really needed, but nice insurance.