Mmmmm ...
I have absolutely no experience with the 109 .... BUT heaps of hours on the 412 ...
IIRC there was a situation with the 412 some time ago where certain Elastomerics were discovered to be made of slightly differing compounds which led to some interesting tracking and balancing issues (
) ...
Visual inspections tended to mean very little as the elasto's always looked perfect ... check records ... did this issue spring up after the changing of one elasto component in the head?
If all other methods of rectifying this vibe fail ... I'd suggest changing ALL the elasto's as a 'complete set' (I know this will be expensive) and seeing if the problem dissapears.
Good luck ...