We found on one car, 66 vette, with a keith black 502 built engine, the surging was coming from, restricted exhaust flow.
he installed dynomax mufflers and all the problems started.
Car accelerates ok, 2500-3000 car seems to surge in power.
removed the mufflers and ran it, problem was no longer there.
Strangely enough.
Just thought you might want to look into it.
more flow more power more exhaust.