If it's not an ignition or fuel problem, or valve adjustment, the next thing to check is the condition of the cam and lifters. To do this, take the valve cover off and turn the engine over by hand. Measure how much each pushrod rises---they should all be the same. If one is less than the others, that lobe is worn, and you need a cam and lifters.

Another thing to look into is valve seat recession. This is a result of using unleaded gas in an engine with a plain cast iron head. To check, remove the rocker arm shaft and lay a straightedge across the tops of all the valve stems. They should all be even with each other. If one sticks up, the seat is receding, and the cure is to remove the head and have hardened steel seats and new valves installed (any good machine shop can handle this)

If the valve seats are good, check the compression of the engine. If they aren't, remove the head and examine the cylinder walls. They should show traces of a diagonal crosshatching pattern. If they are polished completely smooth, they are worn. There should be little or no ridge at the tops of the bores. A thick ridge indicates heavy wear. If the compression is very low or uneven from cylinder to cylinder, or the cylinder walls are badly worn, it's time for a rebuild or replacement. If these are OK, all you need is a valve job (with hardened seats) or a cam and lifters. It is entirely possible you could need both the valves and the cam, but the bottom end could be OK.