There is no diaphragm that can cause a vacuum leak. Bad gaskets could. There is a spring in there, that if it breaks, the throttle plate will close too easily and won't rev up. It sounds like maybe the fuel filter, located between the tank and the lock off solenoid, could be plugged enough to be restricting the fuel flow. The flow may be good enough for idle but when throttled up the flow may not be good enough and the pressure starts to drop in the hose downstream of the filter and after 20-30-60 seconds it goes lean and stalls. Also, I would check the PVC valve.