It sounds like fuel delivery,
carb air, or engine temperature.

Maybe the vent tube inside the
fuel tank is pressing up against
the top. Maybe the vent line is
kinked behind the firewall. Maybe
there is something in the muffler
pressure nipple.

Maybe the klunk I hitting the back
of the tank. Maybe the fuel line is
kinked behind the firewall. Maybe
the needle extension is getting
pushed by the cowl.

Maybe there is not enough air entering
the engine compartment. Air passing
over the cowl draws air out. If not enough
enters, it causes a partial vacuum inside
the cowl making the engine run slower.

The same thing could cause the engine
to get hot and slow down. Heat related
surging has a slower cycle than air pressure
related surging.

Jenny

Edit: Sorry I misread your post. I thought
you said it was ok on the bench but surged
in the air. Since you are still working on
bench running, disregard (but don't forget)
the stuff about cowling.