NiCad help
It is unlikely that you ruined them by "topping them off" a few times. More than likely, the NiCd went bad (if it is bad) by itself over time.
Try charging them up again. Perhaps the 6-cell pack would never work well in your setup.
Today, those cells are simply not used except for in Great Planes ElectriFly products. I would recommend replacing them with 300mAh NiMH cells that have a similar weight.
Good luck!