A change in EGT means a change in air to fuel ratio, have you checked closely for air/ vaccum leaks? I had a slight vibration ages ago when I owned an O320 but only at certain rpm's, was a split induction tube hose! Two plugs per cylinder not only gives redundancy it also improve the flame front across the entire piston face for better combustion which in turn effects EGT's.
Hope you can get the Rotax gurus on to it👍