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You've done the voltage check on cranking at the battery. Try doing the same thing at the starter and see if you get a much bigger voltage drop. The going dark tends to indicate a big voltage drop which means high resistance somewhere. Could be a...

Made the mistake once of being a bit heavy handed with a jet wash and blasted the pressure transducer. It was quite temperamental after that. If I remember there was what looked like a ring of silicone that I blasted off which obviously let water in.