You would need a somewhat expensive digital peak charger with discharge function in order to properly discharge the battery pack. In the USA, a popular choice would be the Great Planes Triton. However, it probably costs more than your entire setup...

I would try charging the pack using the at-home slow charger, following the included instructions to the letter, one more time. If the power doesn't improve, then contact the manufacturer/distributor of the Electrafun and either get a replacement or your money back.