[snap]

Probably a sevo or program problem. I broke the nose of mine three times in two days glued it back and it still flew straight. My friend had a similer problem and he accibently hit a switch that added mix to the plane. But my guess is that when the bad servo is under load its creating blowback wich will give you a pretty bad couple when under pressure. Try doing a gentle light big loop it should reduce the effect, the harder you pull the worst it will get if that's the issue.