Stalling Problems

My mother has a 1990 Sunbird that has been having problems with. It started a few years ago when it stalled at a light after 30 mins of driving. It would fire up fine, but stall again after putting it into any gear, excpet Neutral. After that it ran fine for about 1 1/2 years, it did a lot of highway miles. Now the car is doing the same thing, I have changed the fuel filter, could it be a O2 sensor?

i had the same problem and my uncle does transmossion work and he told me to disconect the electrcial to the tranny so i did and it worked