Vacuum leak, it was not.

[Bruce Edwards]

What does your tach show? Does it show a break down by bouncing when the engine is missing?
If not sounds like Pete is closer to the mark. Do you know how to "Read" spark plugs? Take the car for a run at the speed the problem shows up. Pull the plugs and check to see what they look like. If they show a lean mix up the jets.
I am a Holley man after fighting more Edelbrock carbs than I can count I do not use them any more. It does sound like a lean mix problem that I have had with other engines that customers demanded I use a AFB on. I had one that would run fine at speed but would not idle. After swapping rods and jets it would idle but the cruise speed had a miss. Never could come up with the right set of rods and jets to fix the problem and I have three or four of the Edelbrock "Tune" kits and still never seem to have the right ones. After hours of swaping and testing I called the customer up and made him a deal
I told him I would by a Holley for it our of my pocket and put it on the car out of the box. If the car did not run any better I would keep working on the Edelbrock at no charge to him. How ever if it ran like I know it was set up to since I did the engine he would owe me the cost of the carb and the back labor to try to sort out the AFB.
He came down I opened a sealed box and dropped on the Holley and told him after I set the idle mix I stood back and told hime to go drive the car.
He came back with his check book out a big smile on his face and a hammer for the AFB.