I`ll bet that you have the 29D type distributor ,,, ???
with the vac advance hooked up,,,???
& the base plate is badly worn causing the points to open more, open less,, open more, open less etc etc etc
as the vac advance mech pulls on the points plate it cocks it, (retards & advances the ign timing) as the revs fall, releasing the plate back down effectivly making a smaller points gap, hence revs raise once again (advances the ign timing) ,,, & so on & so forth
Just a wild guess tho

cause i have absolutely no idea how things work
But if you`d like to test my theory,,, remove the vac advance,,, Plug the hole in the carb,,, (don`t worry about plugging the hole in the dissy,,, no need to for the time being) set the points gap to where it`s supposed to be (Approx 15thou) --- (making sure they`re in good condition while you`re there obviously) & then set the ign timing to the correct position (maybe about 5 or 7 deg BTDC)
& then go for a run
If i`m right then because the vac advance mech isn`t "acting" upon the worn dist base plate,,, so it shouldn`t surge
Then remove your carby plug from the vac advance port,,, re-attach the vac advance hose & go for another run,,, if it surges like you have described again,,, then you need to either fix the dist base plate or get a new-er (or another) good distributor
easy-peasy