First thing I would check is the balance of your cells with a balancer other than the E-Flite one. Some people who have noticed a similar problem found that it was unbalanced cells, even with the stock charger. I use an Astro Blinky.
Check post #17 in this thread. That might be it also. Proportional in the 4-1.
Also, check your blades to ensure they're symetry is correct (if you have spares).
Are you using full throttle to get the initial lift? If so, possibly the cells are unbalanced. Maybe just take a charged pack down to your LHS (I always use a safety container to transport, and don't leave it in your vehicle) and ask them to hook up to a Blinky (maybe say I might need to by a cell balancer). Right away they'll be able to tell you if it's unbalanced.