Bad brake shaking on '66 coupe

I am rebuilding a '66 coupe with my son. When we got it, needed complete replacement of brakes. My son will be driving it so I did a complete 4 wheel brake job. The issue is when I stop the car from almost any speed, the front end vibrates violently. As you approach 0 speed, you can feel a 'grab and then release'. My first inclination was that I had bad drums. THe issue gets worse with more stops (heat related).

When I first did the brakes, I left the original master in place and have since replaced it as part of a dual reservoir/power conversion. THe problem existed prior to conversion (but after brake job). I replaced all four wheel cylinders, hardware, shoes, drums, flexible and steel brake lines.

After fighting with this for about a month I finally decided to replace the front drums (again - less than 40 miles on them). The problem went away. I drove the car for the next several days, 20-30 miles and they worked fine. After about 3 weeks, the problem has returned. The first stop of the day is fine, then once the drums heat even slightly, the vibration returns.

I've verified that the pads are installed correctly (primary shoe on front) but I can't figure this one out. Any thoughts out there?