Assuming you have a squeaky clean revolver and that you've made sure the rod is straight, I'd look at the tip of the rod itself as the source of the problem.
When a revolver is hard to open, randomly, it is usually not truly a "random" event. You will typically find one cylinder position where it will always be difficult and on the others it will open smoothly. Try to isolate which one it is by marking one of the chambers with something non-permanent. Line that chamber up with the bore and open the cylinder, repeat several times. Do this for each of the other five cylinders. If you find that binding is present only in one position, your problem is probably that the tip of the ejector rod is not "square" and the high side is binding behind the locking bolt (the spring-loaded part atached to the barrel).
Some very light strokes with a good file should take care of this.