how many amps are you charging them at?
if the cells are not hot, you could try and hook up a multimeter onto the batteries and see if there are any big spikes...and check each cells voltage...but since all your batts are doing it, it doesn't sound like a bad cell.
could also be your charger.. mabey try another and see if it stops? I think its nothing more then a loose connection or bad solder somewhere.
I mean, where is it sparking, the ends of the cells, or the connector you used? sounds like theres a short somewhere.