Bad plug can and will do it.
Bad distributor pick up can do it.
Weak or water in fuel can do it.
As Sutphen30 said, valve not seating can definitely do it. So could a broken valvespring.

As with all driveability issues, basics must be done/checked first. People's minds like to scatter and lose focus before this happens.

Intake valve hanging up intermittently at idle. Does it miss if you bring up the rpm's?

Does it backfire steady with every engine revolution when running,,or just an occasional backfire.

It misses with engine rev.

A clogged injector will in fact cause that, had one that acted the same way, popping...

If ir backfires thru the intake with every revolution while running you have an ,exhaust valve Not opening,, pull the valve covers ,,check if a rocker arm stud broke or pushrod came off,,, I broke an exhaust rocker arm stud years ago, it popped thru the carb. with every revolution when running,, compression will check out ok because the valve is closed.

Yanking the heads asap. Will let you know what I find out. Gonna go ahead and replace the manifold & riser too while apart.

add in collapsed lifter from sitting up over winter valve compressed
or some ex valves got rusted stems and sticking as stated

Turns out I had bad manifold that leaked over the winter. When I started it up, had stuck valve which caused bent pushrod and a few other bad things to happen.

I would take a shot at freeing up the valve and doing a compression test.