Sometimes it is something really simple, like a spark plug wire causing an arc.

I have not done this in years, but it can be effective. Put car in garage or in driveway, but at night, you want pitch black, no lights on in garage or in front of garage. Lift up hood, start car and carefully look for arcing at coil, power stage, each spark plug wire, well you get the idea, anything that delivers power to the spark plugs.

Fixed my old 240 a couple of times that way by finding the coil arcing every now and then. You may need to repeat the test a few times. Good luck.