I was just going to post the very same thing - good job i read all the posts first..

Check/clean the loom earth tag connections (brown wires) on the plenum chamber, but ideally make up a stout earth lead to go directly from the battery neg terminal to the plenum chamber where the others bolt on.

Sorry Pete - didn't really think about these issues talking to you yesterday.. too busy thinking about my own I guess..
It's the nutty tacho bit that gives this problem away.