Hi Rich I agree with you're theory either the coils, maybe hairline crack opening when warm or as you also hit on the ecu can have an intermittent fault as I've had in the past, you should be able to find on eBay, l assume you've already tried the easy stuff first like plug, leads, caps. I think the only way to test ecu is to try another if you can borrow one but I think its best to have a spare if possible anyway as the do tend to start breaking down around the thirty year old mark. With the coils you could try switching them over to see if she still dies on same cylinders.