Wow man, 5sec is a LONG time to have anything hot near the A123 cells lol. You should be using a really hot iron, and literally just touching it on there to solder ie <2sec. They get damaged pretty easily by heat from soldering.

As for the A123s keeping in balance, they do to a certain extent. There's no real point in my experience to slow charge/discharge them. All my packs remain in balance fairly well, unless you discharge lower than like 2.2v/cell, then they go out of balance real quick.

Normally, if you just hammer them with high amp charge they normalize and come back into balance near the end of the charge, but only if they're not really too far out of balance to start off with. As I abuse my packs in my vehicles, I've got balance taps on all of them so I can balance charge when needed.

For your particular situation, I'd suggest charging each cell up so you're sure they're all individually charged to the same voltage (this is if you don't have balance taps obviously) and then discharge them all together, either on the car, or on a charger. Set the LVC for 2v/cell just to see what happens. If you have a balance adapter you can monitor the cell voltages, and you'll see straight away when the suspect cells begin to dump voltage, which they'll do REALLY quick at somewhere near the bottom of the discharge.

If they do that, then once the pack is discharged, when you charge each cell up individually, have a look at how much mAh is put back into each. The broken cells (if they are broken) will take far less that 2300mAh back.

Hope that helps
-Sid