For sure being able to check for spark is a positive as far as diagnostics go. When it wont start and it for sure has no spark you will then know it was not fuel. Then again if it does have spark its on to plan B. One last item to try when it is acting up is the jumper from the battery to the coil positive. That 9 volts is at the minimum . You could also make up a jumper for that modified ballast just in case it is goofy. Your voltmeter will tell you the story. I have seen modules fail unpredictably.