Paul, seems that you may have the common connected on the opposite side lug to where it should be. The common will be the wire wound on the coil which of a thicker gauge.

I didn't have time when posting last night to clarify what I had said in the previous post. There are 2 sets of winding on the coil. The common lug will be the one where the 2 sets of windings on the coil connect (One thick the other of thinner gauge). What I believe is happening is your common wire is only connecting to one set of windings through the EOS. When pushing the flipper button, the coil on the EOS circuit is energising & immediately de-energising creating arching and chattering.