Had the exact same symptoms on my 394 Oldsmobile motor with quadrajet. Travelled great on the freeway but at idle stopped at lights and behaved as yours did. Just made it home, i suspected it was the Carby sucking air as this was a common problem with the quadrajet on that motor. Just so happened to have a brand new Holley 600 in a box in the shed. Changed it over, I still have that Holley on and running great.