Infamous P171 & P174 - MAS replaced - PCV elbow replaced

I have a 2001 Mercury Sable LS. A problem started back about 5 months ago. I would start the car with no problems. The car would go thru about a 2 min cycle on a higher idle and then when it was over the rpms would drop down to 500. It would act like there was no gas and jerk or lunge causing lights to dim and brighten because the engine was surging on and off. Finally it seemed like the computer figured out a problem and would increase gas and the rpms would go back up. The check engine light didn't appear for a couple of weeks. When it did it showed the infamous P0171 & P0174 bank 1 and 2 lean. I replaced the Mass Air Flow sensor and it did nothing. Then I saw a posting on another site about the PCV valve elbow where it goes into the intake manifold. The elbow is smooth rubber and collapses causing vacuum problems. Sure enough mine was totally collapsed. The part was only 25.00 at the Ford Dealership and I had it put on. It actually seemed better for a while. I thought that would be the end of it. But about 1 week after the codes reappeared and the same symptoms as well. All this time since it started back 5 months ago I've had horrible gas mileage. I wish there was a way to check the whole vacuum system for problems because that is where seems to still be after reading tons of postings. I'm not sure if it is in the intake manifold gasket itself or some vacuum line. I'm frustrated and very leery of replacing 02 sensors because it seems unlikely that they are both bad at the same time. I also notice that when the car a going down the road I can feel it speed up a little and come back down. It sure seems to be an air/fuel mixture problem. Anybody have and feedback. thanks in advance