That "got progressively worse" comment gives me the impression your spark is going somewhere it shouldn't.
I have seen coil wires burn the center out of them and gradually the spark has to jump so far it won't any longer as cylinder pressure rises.
As the cylinder pressure rises the spark chooses another path (like out of the coil tower to ground or something like that.)
As the gap burns out longer inside the wire it does it sooner and sooner in the throttle because it takes less and less pressure to let the spark go somewhere easier than jumping that gap.
Point is, a burning down the center coil wire acts exactly like what you describe.
It definitely sounds like an ignition issue.
One big clue is that an engine with this problem will backfire through the exhaust AS SOON as the throttle backs off and the cylinder pressure goes down allowing the spark to jump across again igniting the fuel in the exhaust system.
Just a thought because I have seen it a few times before.