Once you get the car in gear, do you still have to "two-foot" it?

I had very similar symptoms, with the engine also lungeing from about 400 rpm to 1200 rpm every couple of seconds when in gear. I solved it by changing my mechanical advance curve to have it start at about 1200 rpm, have a total of 22 degrees in by 2500 rpm, and set the initial timing to about 12-14 degrees. The vacuum advance was hooked up to ported vacuum, not manifold vacuum.

My problem appeared when I put in the 041 cam (231/240 degrees @ .050"). Yours has enough duration and overlap to be the culprit. If you haven't set up your distributor similar to this, that would be the first place to start IMHO.