Did you try replacing the distributor cap and rotor? Sometimes the rotor will "leak" and you won't get a full spark to the plugs. Also look closely at the coil wire. The way it's positioned lends to a great potential for it to leak voltage too. Look for "whitish" bands along the length of the coil wire from the coil to the distributor. If you shee several places that aren't consistent with the black wire replace it, probably if you see that it's time for a complete tune up with plugs cap rotor and wires.