Ok, so I've been trying to diagnose why I get intermittent thumps in my
exhaust.

Today, I put my timing light to all 4 ignition wires, and found that 1 and 4
do not get spark once in awhile. Sometimes, #1 would miss for a few revs, then
I'd hear pop in the exhaust.. #4 doesn't miss as frequently. #2 and #3
don't miss at all (or barely noticeable).

Anyway, I'm guessing the thumps are caused by series of misfires, afterwhich,
fuel in the exhaust ignite.

Funny thing is that when I pop the hood, and when the engine bay cools down,
the misfires go away. Does igniter performance degrade with heatsoak?