DJ, off idle, at "running speeds" your vacuum leak will not make near as much of a difference as it does at an idle. 52Carl is correct, you need to plug that line and get it running properly at an idle with that plugged. Once that is done, and timing checked/set, then , as Dave said, look for soft motor mounts. Soft mounts may allow the engine to rock, moving the throttle linkage, (same as you pressing on the gas) then it will lurch and move the throttle linkage the other way (like letting off the gas) and keep going back and forth.
