Electronic ignition. Mallory Unilite distributor, exact same as mine, same problem, searched long for that one. In my case the electronic ignition module was bad. And there is a test for that.
http://www.manualsdir.com/manuals/337302/mallory-ignition-mallory-unilite-test-procedure-mallory-unilite-test-procedure-605.html
Check it out, highly likely that is also your problem.