Your NiMH battery is probably the problem. I had one brand of NiMH that caused my problems that every time I let the pack sit for a couple of months, one cell would die. The charger thought everything was OK and it would show a voltage you'd expect while charging, but if I used a voltmeter afterwards, I'd see the total pack voltage was only around 8v, when it should have been over 9v.

You might have multiple bad cells. Might want to test voltages of every cell to see if they are similar.

Not uncommon for NiMH batteries to get really hot after running. The older they get the more internal resistance they develop. I'm sure you're not using your Rusty for a Sunday drive. Used to run a Rustler with a modified brushed motor and batteries would come out of it after a 5 min race too hot to handle.