I personally wouldn't use them like that, as mentioned earlier you should at least re-wrap them with a new insulator on the one.
The fact that your mod was acting up would be the first hint that something is not right.
I've had considerable experience with different batteries from the RC Car world.
From my experience, anytime a battery looks odd or starts acting strange I just get rid of them. The price of batteries is cheap compared to losing a couple of fingers or worse.
Better to be safe than sorry.