Your braking power may also be lower with bad cells, because the controller uses the batteries as a resistor to brake (the higher the internal resistance of the cell, the lower the braking power)
Maybe
this will help you mounting your batteries tight? I'm sure Mike can get them as well.
The powerswitch you'r planning on using needs to be in the receiver path, don't put a switch in the powerpath (batteries/controller) this can hurt your controller.