Hi,
I came across this one a while back myself and asked the same question.
A helpful RCU member suggested I check the flybar as it may be bent.
It looked ok but I took it off anyway and rolled it on a flat surface and it was indeed bent.
I managed to straighten mine and the problem was solved.
I was flying a lama XRB contrarotating heli, it may be very simular to yours.
Hope this helps.