I put a splice to ground for both ECU ground wires and it seems to have the engine running as it should, I can move the ECU around a little bit now and it doesn't affect idle, it does highlight the next symptom now which looks like the twisted pair wiring for the front speed sensor. Moving them around with the engine running does cause speed fluctuation.
I wasn't expecting that, does it need to routed well away from other high current wires, or perhaps there's a problem with it too?
Or maybe it'll be ok when I tape the wires back up.
Hmmmm...