One condition that has not been addressed is the possibility of valve timing being off. If the cam gear is worn out it may have possibly "jumped" a tooth or two and the cam may be out of sync with the crankshaft. I have experienced this with high mileage engines. The fact that the valve train is operating is no guarantee that the valves are in time with the position of the pistons.
Check the position of the valves by removing the valve cover, while observing the position of the 1 piston position in relation to the timing mark on the flywheel and the position of the rotor and the position of both intake and exhaust valves. Both valves should be fully closed at TDC with the rotor pointing to number 1 secondary terminal inside the distributor cap.
