RPM stability in a GM HEI distributor is a function of the crappy pickup coil/reluctor setup that they use. The HEI module itself is stable, a long as you use a quality part. My personal bias against its use has to do with two things:1) It has a variable dwell that is supposed to climb from about 15 degrees at idle, to about 38 degrees at full saturation. A MOPAR electronic ignition saturates the coil at 40 degrees dwell, from start to top rpm. 2) It's GM. 'Nuff said? Asfor E-core coils, an excellent one is stock on later 4- and 6-cylinder Chrysler products, and gives excellent results. Pulled mine from a junkyard caravan and haven't looked back.