Both Gavin and Jmac methods are correct... the resistance method is a quick way to check the continuity of the connection, if you find an issue, Jmac method will pin point it

I would remake all the earth connections...clean any ingress and crap you might find and clean up all the loose wiring in the engine bay making sure that some frayed wire isn't earthed, just to be certain and you would be glad you did it. Failing that, I would most likely say that the issue is with the ECU.... did you get hold of the other ECU you mentioned earlier?