I wonder if it is just a battery ending its life, I would start with a new battery if yours is over 3 yrs old or if the car has not been used a lot, then clean all the earths and maybe send off the ecu to be checked by MSD. The looms in the engine are known to fail with age so that would be a good swap out and not too hard to do (you might even persuade Rod Tarry to build one)