Lower your head speed. dial in yourgyroagain to wher its a rock solid holding. ( gp780 is an excellent gyro) And from ther just tweak your head speed back up and gyro settings to match.
There is no reason to lower your headspeed. If you do that, then your setting might be off again when you bring it back up. I still don't think much of the GP780 from my experience. BTW, what tail servo are you running?