Fuel pump, oxygen sensor, or other?

I bought a car from a guy that said it only
needed an o2 sensor. It idles rough on
start, then smooths out once it's in gear.
My friend noticed there was a vacuum line
from the intake that lead nowhere and had
a screw jammed in the end where it was
supposed to meet a "t" section, so we
replaced that now since that and checking
the plugs the ses light has been flashing
once it gets above 30 mph.
Yesterday on the way to an interview I put
the car in reverse and it acted like it
wanted to die, so driving down the road I
stop at the intersection and the car just
dies and won't start. It wants to turn over
and even starts but then immediately dies
after.
Just wondering if the o2 sensor can cause
this or is it more likely the fuel pump or
something else?

Car is a 2004 Pontiac Grand Am 3.4 L V6