Nimh discharging and charging issue

[Snelo]

I have recently purchased two 4 cell 5000 mah nimh battery packs for mardave racing. One of the batteries works fine. The other battery when charging does not cut off. So just constantly charges until gets hot and I stop it charging. I have tried discharging however it discharges down but when nearly finishes the voltage starts slowly going back up again as if on a slow charge. The charger still says that it is discharging.

I am using a core rc uac 40 charger, it works fine with other batteries.

Any ideas why the battery is doing this?

Thanks.