I have a few packs that do this on several different chargers, and it is because they are not balanced well or a cell is getting weak. I cant say this is what is going on here, but it has been the case for me. One way to know is to stand there and watch the individual cell voltage. Every few minutes the charger should allow the pack to "rest" for a few seconds while it checks voltage, and at this point you can see one cell voltage shoot up and the other stay the same or even drop, depending on how bad it is out of balance.