Nope, just after every weekend, I take each battery and charge/balance them [individually] at like 1.5A.

I doubt there would be anything else doing that. Either that, or your charger has a plus/minus 0.2v tolerance, or something along those lines... Regardless, there shouldn't be anything pulling more juice from one cell in a pack, than the others, IMHO.

Out of shear curiosity, what kind of ESC do you have? and how old are your batteries?