I observed that gps position reported in MP wonders on my arducopter V3.0.1 with Ublox neo6M.
I tested another board with another gps and i got the same wondering of gps lat long parameters in MP.
Then with APM connected, i unplugged the GPS and to my surprize, the lat long parameters continued to update on a curved pattern, further away from home point, even a No GPS text was displayed on MP.
After a reset everything ent back to normal but when i loaded previous parameters, the same error happened,
That beying said, i rolled back to 2.9.1b until this issue is solved 
After installing gelly vibration dampers, the copter behaves pretty well with AC3: