It could be your jetting, a little on the big side? Does the engine start to make the droning sound as you try to accelerate hard and then begin spluttering ('dut, dut dut, dut' engine sound)?

Sometimes as you accelerate under load (especially in the higher gears) it'll start to hate it (richen right up) and begin spluttering, sometime you have to clutch in and rev it to burn the excess and then it goes okay again until it over fuels itself again.

Me and Gylezy have this issue with our carbs fueling on main circuit when under load in high gears and low to moderate revs.

Never heard of the stock pumps running out of wind (except in race applications and different fuels) unless, like you said fuel lines or facet diaphragm or spring have failed. Otherwise jam an exhaust gas reader up the exhaust and see what it's doing?