Don't forget: Wheel balance and runout. I had imbalance cause this on one of my my race bikes. You could feel a somewhat gentle once-per rev shudder that increased with speed, but once you put the brake on it got really bad.
Check for drum distortion. Get the front wheel off the ground. Gradually tighten the brake adjuster as you rotate the wheel. They generally will have a gentle tight spot, but if can rotate it a ways and then can't rotate anymore, your drum is warped bad. Get another one or get this one turned.
Cheers.