Pontiac Bonneville runs bad then dies

My Bonneville will start and run like a top for a
short time then will bog down, chug and die. If i try
to start it after it dies, i have to pump the gas to
keep it going and it runs really bad when i do with
huge clouds of weird smelling exhaust.
Sometimes if i continue to pump the gas i can get
the exhaust to subside and the car to idle normally
for a bit until it dies again a short time later and the
process starts over. I can hear the fuel pump
engage when the key is turned on. I took it to a
mechanic and he replaced the mass air flow
sensor but that did nothing except leave me
stranded on the side of the road minutes after
leaving the shop.