Broken hold winding would do that.
The flipper chatter as you call it, is a sign that your eos works correctly.
Replace the coil or look for a broken wire at the connections of the thin copper wires.
Sometimes you can find it and re-attach it to the lugs.

Peter
www.inkochnito.nl

Inkochnito is probably right but you can try swapping coils to make sure.

A couple of things- the wrong type coil stop is installed, dirty and/or misadjusted eos switches. There should be a 1/8 inch gap at the eos switch contacts at the end of flipper travel.