Girlfriend's 2002 Yukon will crank but then occasionally RPM's will drop and it will almost die. Replaced O2 sensors but ... still occurring?

What condition is the battery?

Your car cold start valve puts more gas into the system keeping your engine running higher RPM. When you car warms up, the cold start valve will stop working and your car RPM will drop. What you could have is a vacuum hose disconnected which puts more air into your engine - therefore when you press on the gas pedal, your engine would shut off or you find lack of power and then shut off.

Is the check engine light on ? Sounds like you are loosing spark and getting it back.

it doesnot crank

crank sensor will cause similar stalling issues. You may have got a bad alternator, if the battery light is on after you jump it, probably needs a new, new alternator. make sure all your connections are tight and plugged in, both on battery and alternator.

sounds like the battery has a dead cell in it. i would get a new battery.

Check for vacuum leak at intake or plenumn while it is cold , also check temp sensor. i have done a few intake manifold gaskets on these vehicles.

Check for vacuum hose leaks or component clogs. You can use vacuum pressure tester.
or turn the idle screw down.

Change the coil pack if your getting No spark.Or the crankshaft sensor.

remove the aircleaner intake hose and, check the intake air control flap for free movement