Mmmmm ......
One of the many issues the years have thrown at the 412EP .... have you investigated the matching of the M/R Elastomerics ... and at what Nr you balance the M/R in cruise ... in the past we (?) have noted that depending on the age that they can cause issues especially when lifting HEAVY loads (especially heavy sling loads) ... if the RPMs are not beeped to say 101-102 the coning angle can allow for a sort of 'Hooks joint effect' oscillation which the engines then try to catch ... the engines the have a fight with the Yaw SAS and you can end up with an O'torque which spoils all your fun .... turning OFF the Yaw Ch. reduces it .... also putting the T/R into clear air when lifting also helps (but brings you into conflict with the RFM re: wind from the right etc etc) ..... compromise maybe the key (as Bell did with the design of that M/R head) ... *sigh
Cheers
Spinny