Yeah, I'd say something wasn't quite right with your old setup. 3mph is substantual.
I have an old mallory dual point too, triggering a Crane HI-6M. I just got it running and I'd say something is screwed up. It's OK until, say, 3,000RPM, then it gets a mean miss. The tach reads WAY low too so I'm thinking the box isn't getting a clean trigger signal.