at speed things are fine, but a few sharp turns slows it down much quicker than other Vipers turning even sharper and going slower and eventully leads to it stalling compleatly and falling from the sky in a rocking fashon.
If you've got the equivalent of a second battery pack worth of lead in the nose, the extra weight would explain why you slow down so quickly, but not why it's unflyable without the lead. If it flys fine at speed with the nose weight, maybe try shifting the actual battery pack itself so you need less extra weight.
Hate to say it, but from what you've described I kinda agree with probligo; it sounds like a problem with the entire airframe. I don't have much experience with EPP foam, could one wing be very much denser then the other?
In straight and level flight, with no nose weight, what control input do you have to give it to keep it flying level? What happens if you give it no input?