If you need a capacitor there is something else wrong. Generally poor grounding either Battery negative as in ground loops poor connections or wire size. You can also have poor grounding inside the ecu. Any system that uses battery - as a sensor return wire would be suspect. Well designed systems have an internal ground plane isolated from battery- for sensor returns(ground sort of). You might also check the altenator as bad diodes will cause all sorts of problems most suspect or me are single wire import ones.
I had a fast system I was called to fix that was firing the ignition with key on engine off and not just one you could hear the coil buzzing away pretty shakey setup in my mind.
Dave
