During one of the recent lock-downs I decided to replace distributor caps, spark plugs and leads in three of my 360s. In two of cars the plug leads were original and let moisture in the system after months of no use. Then with the new gear they all began to play up to some degree, like hard to start and running on a few cylinders for a few minutes. Also two of them would run rough and backfire after about twenty minutes on the freeway.
I have never experienced this sort of problem during my sixty years of frigging around with cars.
Obviously the newly installed gear was the last to be tested. There was plenty of sparks at end of the leads, plugs sparked ok as well. Then I measured the individual cylinder exhaust manifold temperature with an infrared thermometer, all about the same temp. Drove the car around the block a few times, all ok. But next day after a cold frosty night the bugger did not start properly and ran on a few cylinders. Using the temp gauge I immediately found the problem cylinders. On the cylinders with liw readings I found that the plug leads had become slightly loose, enough to make for a dodgy spark. The problem is that the lead/spark plug cap is too tight and too long, not letting the lead clip to be pushed on to the plug properly, if at all. I suspect the leads slowly worked their way off the spark plugs as they had become loose, sort of squeezed off.
Remedy was to cut about 12mm off the plug cover allowing the lead to be snapped on to the plug correctly.
Next day before starting the car I decided to clean the engine room with a high pressure hose as the engine had become grubby. With a can of "Start-U-Bastard" at the ready I turned the key and immediately the engine idled smoothly. likewise after the spark lead/plug covers in the two other 360s had surgery there has not been a problem since.
Laurie