It's long since time for plugs and wires, however, if the ECU does not recognize any misfires, you might try a vacuum leak test:

Start engine.
Raise hood.
Observe how "shaky" the engine is.
Yank oil filler cap off.
............... Did the engine get even more "shaky"? If so: Vacuum leak! (FIX IT.)
............... Did the engine get even more "shaky"? If not: You have no vacuum leak. (Keep looking for some other cause.)