The variation in the timing could be causing the variation in the RPM . . . or the variation in the RPM could be causing the variation in the timing. I wouldn't think the coil is the culprit. My first suspect would be the distributor bushings worn, but if you have a new distributor, that "shouldn't" be the problem . . . but quality control in the automotive parts industry isn't what it once was. I have gotten my fair share of "good, name brand" parts that were bad right of the box.
When you check the timing, are you checking it at idle or at full advance? At anything less than full advance, the centrifical and vacuum advances can move around some and cause the mark to dance around several degrees, but once you hit the full advance, with a new electronic ignition distributor, it shouldn't dance around any. Under full advance, it should be set to fire at 32 degrees before top dead center.
