I think you have coil packs per plug. Could one be faulty & engine temp resolve that as it gets warm? What do Ford forums tell you
Thanks John that's also a plausible scenario.
Unfortunately car has now developed another more troubling fault: it won't start -- makes a racket when I turn the key, lounder than a simple solonoid/relay click (if relays even click?), kind of like a bb gun firing at about 4 shots per second, so trying to get that sorted is now job 1.
