[cdn914]

I have a completely stock 914-6 with a persistent problem - popping from the tailpipe and missing intermittently. I have changed every ignition component including the ignition switch and distributor, and even the carburetors but the exact same problem persists. I have substituted four CDS units and the problem is exactly the same with all four. However, none of the four CD units is new. All emit a hum with the ignition on and the car does run. How would I know if the problem is the CD unit? Can they be tested and if so, how? Do they completely fail (car will not run at all), or can an intermittent problem be caused by a bad CD (or in this case four bad CD units)?
Is there a better alternative to the CD ignition? Any and all suggestions very welcome.