IMHO,that is a power piston. it does the transition from idle circut to power/cruz mode.

goes up with less vac to raise the needles out of the jet to flow more fuel. goes down under high vac to lower needles in the jets to restrict fuel flow to the idle circut.

power valve will have a rubber diaphram(sp?) that will ruptur and dump fuel all of the time makeing for a rich idle and slugish throttle responce from a rich idle to a rich power/cruz mode.

power valve should only open under low vac rating to allow more fule to flow at a power mode/WOT.

the bore for the power piston carbons up and causees it to stick sometimes in the up or down to cause drivabilty problems like you discribe.

like the other poster mentioned,it may have a retainer poped loose and would need to be restaked to hold the piston in the bore corectly and right hight on needles/jets.

I have to restake the retainer on the power piston on my 40 yr old q-jet now and then.