As Phil alludes to, the issue may be at the butterfly connection to the throttle shaft (or the darn shaft/bush in the carb body). The best way to check the shaft is the old flammable gas/liquid squirted on the outside of the shaft and listen to the engine speed. Any change (up or down, depending on various factors) means a leak.
The butterfly can only be checked by removing the carbs and physically checking the seal when the are supposedly "closed." Also check that the overrun valves (if equipped) work OK. Not stuck, spring not missing, not too gunked up.

Sometimes the linkage between the carbs can push against the shafts and cause them to put undue lateral pressure against the butterfly and they stick or wear against the side of the carb. Loosening the interconnect levers and pulling them away from the carbs helps unless the shaft is slightly too long in which case you either have to shorten the shaft or loosen the carbs and retighten tehm to the manifold while applying pressure to the outboard to give a little more room.

Ongoing implies that there is a physical issue (misalignment) with the carb/manifold as opposed to general wear or something having broken.

Last place to look is the plugs in the manifold. They could be faulty (somehow) and allowing a leak. I recall someone who had pressed in plugs that would move as manifold vaccum increased and decreased and it couldn't be diagnosed until he actually poked at them with a screwdriver.

Mike!