If a different injector made no impact, then you need to check the electrical pulses to that injector. If the problem is electrical, it is most likely the wire or connector.

Have you considered the possibliy of carbon buildup on the exhaust valve? That would be consistant with the symptoms you are seeing.

I take it that the car runs on 3 all the time, not just at idle, right?

One last thought - remember that it is harder to fire a plug in the hole than it is to jump a spark in ambient air. Check under the distributor cap for cracks and carbon tracks.