Spektrum/JR came out with a simple capacitor to address low voltage...

However it is far better just to use a higher voltage pack of higher capacity.

Newer Spektrum/JR units ship with adecuate 6v packs.

There are many options... e.g. LiPo's with regulators, A123's, 5 cell NiMH and 5 cell NiCD's.

I primarily use LiPo's with regulators or 5 cell NiMH packs as both can be had inexpensively if you look.