Sparks

[BrianG]

Yes; sparks, hot batts, and no power are all very good signs of shorted batteries. If you don't want to heat-shrink the pack, then at least use some type of insulating material between the pack and any part of the chassis where the batteries contact (and may contact) - like plastic, rubber, etc.

What is probably happening is the short-circuit is momentary, which is why the power comes and goes and would also explain the occasional sparks. Also check the cell casings. The shell of each cell is the same point at the negative lead so if the vibrations abraded a part of that on two or more cells, you are effectively shorting them out.