2004 Mazda 6 missing cylinder

Engine Performance problem
2004 Mazda 6 6 cyl Two Wheel Drive Automatic

My car sounds like it is missing a cylinder. At first i've been told that it would probable be an injector gloging or failing. I had them all taken out, cleaned and tested and the probem remains. Specially at low rpm and around a speed of 40 miles/hr you feel the rough idling and stuntling.
If it is not the injector what else could it be?
THX