FYI Some of the ballast resistors had different resistance values to match the hotter than stock boxes.
When ever you have trouble with spark, even if changing the ECU box fixes the issue check the reluctor gap in the distributor (0.008-0.012 IIRC) with a brass feeler gauge. I have had intermitant spark due to a larger gap. I have also witnessed a guy who would swap between 2 coils to get the car going again when it was warm....same issue, I told him to check his reluctor gap and that fixed it