There was a thread on here a couple of years ago about range problems. Several people stated that they had had range problems on 5 cell nicad packs that went away by just adding a regulator or dropping down to a 4 cell pack. The problem seemed to be related to the pack being fresh off the charger and thus quite a bit more than 6V.

I know a lot of guys are using unregulated 5 cell packs with no problems, but evidently it can cause problems in some setups. I wouldn't think that a LiPo or LiIon pack regulated at 6V would cause the same range problem, but you might want to try a nicad pack just for grins.