To me it looks as though the cells you bought are hobby grade and the stock cells are industry grade.
The NCR ones you linked are good for about 3A continuous current draw while the stock cells are rated at 15A (when new
http://www.google.com.au/search?q=SE...JIWTiAfwloGwCA). In rc language - you bought cells with a much lower C rating and your sucking the guts out of them.
That's my perspective anyway.