Need Help With Off-Idle Surge

I think you are getting too much vacuum advance. When the throttle is cracked the ported vacuum comes in, the advance speeds up the RPM. When you open the throttle more the vacuum drops reducing advance, and that works. If there is screw in the vacuum advance you can adjust to limit the advance under high vacuum. I adjust by applying vacuum with a mighty-vac, and visually look at the vacuum and the distributor plate movement, as you turn the adjustment. You will then know the direction to turn. Try moving it 2" of Hg and try, increase if not enough. Document where it was, so if you want to go back you can.

Another alternative is to place a slightly stronger weak spring to slow down the mechanical advance at low RPM. If you feel the rotor by twisting and releasing, it should return softly and with certainty. If it is sloppy at the end of return, it may a problem of slop in the various parts.