[zzmac]

Ok, I finally got the car back to the shop to further investigate. In order to eliminate some possibilities he checked the timing and then took a look at my MSD and replaced the bushing in there with a thicker one. Still had hesitation.
Then he increased the timing. Still had hesitation.
Then we tried a stronger spring in carb. Still had hesitation.
Then we tried a lighter spring in carb. Still had hesitation.
Then we ran out of time but he's now confident it is the carb (not ignition or timing).

When I take it back next time he's going to try a different carb to see if it disappears. If so, we'll rebuild the carb again or buy a new one.