You probably have already done this, but make sure all your spark plug wires are tight and none are rubbing or burned.
I had a similar problem in that the 4/4 would run fine, except when one of the wires shorted being too close to something hot (I don't remember what at the moment?) and had its insulation burned away.
Then the car ran lumpy on only 3 cylinders, until the wire was away from what ever it was touching and sparked the 4th cylinder fine.
Cheers,
