I would experience that occasionally myself with a half full battery if I was climbing or putting in a heavy load. What I would do was run until the anomaly, then use the battery in the Tamiyas to finish them off. I didn't really mind it, getting 30 minutes at WOT with the Benidini, Amp and 400s is very reasonable, but adding another 20 to that with use of another smaller battery would be great.

I assume a separate NiCd or NiMH at 7.2 would work too or even a 7.4 lipo?