I think this problem cause because of the bad quality cells of your battery pack. Maybe you can try to rebuid the battery to replace the cells with new ones,but also difficult to make sure the performance of the new cells...

If you dilike wasting time to rebuild, you also can choose to buy a replacement battery, you can go to ebay to buy the replacement batteries or go to this website
www.powerworldhk.org to see. This site operates all kind of power tool replacement batteries like Makita,Dewalt,Bosch,Hitachi ......