You seem to have tried a lot of stuff and so far no smoking gun. To cover all bases, I would take apart the main engine connector (passenger side, under the snorkel), clean the contacts and reseat. At least this would make you know there is a solid connection there. (I had a different misfire issue, this connector was the culprit)not yet but i will when i reinstall them after a thorough cleaning. Putting Seafoam into oil seems to lessen the shuddering so i might be onto something here.