Watch the video below, it is something they need to fix in GP Bikes (extreme wobbling).
https://www.youtube.com/watch?v=FrOjsolroto
Lawson ;D
use clutch
or close the throttle for a hundreth of a sec.
Just to complete the replies above: it is a known problem, introduced with beta5. But it's still a problem: using clutch/closing throttle are workarounds.
Beta6 will take care of this.
MaX.