either fuel or electrical...electrical is the easy stuff to try first (and worth doing if its been a while)

remove the distributor cap and check the rotor arm and the contacts in the cap...if they are worn or pitted, or lots of black in their or 'tracking' black spider web lines then thats a likely culprit...

if so clean it up with a cloth and clean the contacts CAREFULLY with fine sand paper and see if it get a LITTLE better if so thats your cause..if not but its badly worn change it anyway....

remove your leads and clean them with a cloth...leads should be CLEAN to work well....of course removign all your leads also means you check the connections when you put them back...

tip before removeing ANY leads number them with snopake and draw your self a diagram...

if it all looks ok get it in the dark and start it up and check for visible sparks on the outside of the leads - a sure sign of lead breakdown, mist LIGHTLY with water to aid this....

it could also be a faulty injector of course....

if in doubt replace the spark plugs, leads, rotor arm, dizzy cap...and see if it goes away...but you just spent some $