See my post: http://diydrones.com/forum/topics/yaw-and-loiter-issues-2028 There is a video of what it does. It's because my PID's are to high (so i am assuming, it only takes a small bit of forward flight to get it going) but it sounds like what yours is doing. It does this on mine when (STABILIZE_YAW) P = 0.8 and D = 0.16 (double the default). I now am running P = 0.6 and D = 0.12 and it has went away but my yaw is still slower than i like.
I am waiting for my xbee's so i can easily tune this thing.
Does anyone know,
Is there a chance that this is related to the "Return to Land" when the battery hits a specific battery level?
Does anyone know what the voltage trigger level is?
I have been flying with an unknown C rated battery since the beginning (7 months now) It has been cycled at least 100 times. It's a GWS 2200mAh flat rectangular pack. Each time this did occur I was forcing the quad in a forward motion that would require the 2 motors in the back to draw more amps in turn lowering the voltage, if the battery could not keep up with the discharge rate.
I just got some new 2200mAh 30C batteries in so I will give them a whirl.
Thanks again for all the comments...