blindsquirrel how long since the magnetos were serviced?
the magnetos have a set of points inside them. if the little points cam is worn it will gradually send the magneto timing out.
magnetos have an internal E setting which synchronises the points to the magnetic exciting as well as the magneto setup on the back of the engine.
if your maggies have been on for 500 hours get them serviced.
you will be amazed at the difference a set of magnetos in good fettle make.
my engine went from anaemic to book values immediately.