Electronic Distributors From Rick Ehrenberg

More efficient burn, will illustrate shortcomings in mixture
if there is still time left to be producing power it should be used.
what was adequate on the fueling front no longer appears to be adequate with the symptom you mention.

(although air leak at inlet or around exhaust, manifold could cause something similar)

When you replaced the dizzy you
1) probably made better electrical connections
2) altered the timing and the timing curve

so the spark could be better at its job.
and its timed differently at every RPM to what it was before

sounds like more efficient operation that needs some jetting work to make good
or you have a very wrong heat range plug.
or something was disturbed during the work and blocked up a a jet or filter somewhere.