Cuts out at WOT?

This is weird. I have the 9918 + 9L + UBEC on my stock Emaxx. When i gun it WOT in 2nd gear, sometimes it will stop-go-stop-go thru the entire acceleration range, and a little bit at top speed. doesnt do it in 1st gear tho. This is with brand spankin new matched 1.18 GP3300's from maxamps. It does it half the time, which is really odd. It NEVER did this with my old, used 1.17 matched GP3300s (which are dead now). It started doing it when i started using my old 7-cell sanyo 2400's. I figured they being so crappy, that the intense amperage caused the volts to drop dramatically, cutting off power to the uBEC or something. So i get some new batteries, and the problem remains.

There is no drivetrain binding, gear mesh is set properly, and running on 14-cells with deans connecters (all solder joints are what i like to call "perfect" hehe)

Whats wrong with my baby??? :W: