2000 Ford Focus

2000 Ford Focus 4 cyl Automatic 140000 miles

my 2000 ford focus keeps stalling. It only stalls when I stop. The check engine light is on and when I took it to advanced auto they ran the code and it comes up running to lean. So they suggested I get a new 02 sensor. So I replaced that and within a week, same thing. I did come to find out that there is two 02 sensors, but I took it to a dealer anyway and they confirmed that I needed a new 02 sensor(the one i've yet to replace) so I bought an o2 sensor and had it put it. They car ran great for about 3 weeks and now the same thing has happened again, more stalling and the same code. Help?
May 28, 2010.

Lean fuel condition could be caused by one of the following below: 1. Vacuum leaks
2. EGR and PCV valve.
3. Low fuel pressure.
4. Dirty fuel injectors.
5. Mass Air Flow Sensor.

May 30, 2010.