Is the vacuum advance working?  Does it always quit at a certain rpm?

Kinda sounds like a flaky fuel pump. What I would do is pull
the air cleaner covers, etc, and check the little glass window on
the side. Have someone watch that window while you do the gas.
Normally, the level should be at the halfway point, on the dividing
line. See if the level drops way down at the point it's gagging.
If it's the pump, it would drop down low about the time it gags,
but then fill back up to the normal point when idling. The demand
for fuel at idle is low enough for it to keep up with.
If the level stays at halfway no matter what, it's probably something
else.. Filters, etc, cept you say you changed both of those already.
MK

Your other replies overlooked the obvious- clogged catalytic convertor

-SP