[Laird Scooby]

Inlet manifold gasket? Loose inlet manifold? FUBAR Lambda sensor? Split hose between inlet manifold and auxiliary air valve/idle control valve or one of those valves is faulty (you'll have one or t'other, not both), disconnected/split vacuum advance hose, fuel pressure regulator leaking air - could be many things!

Get an assistant to keep the throttle pressed where the engine hunts most and spray all round the top of the inlet manifold (where it joins the head) then the bottom, round the throttle body, round any hoses stubs on the inlet manifold, here there and everywhere until the engine note changes. Now narrows your search down to where exactly you have to spray if for the engine note to change.