I have a 2000 Sable with a 3.0L Duratec 24V engine. The car has approx. 130,000 miles on it. It has been driven with the check engine light on for the past couple of years and had been running perfect up until six months ago. It seemed to be getting progressively worse over the last six months. It was running very rough at idle while in gear. I started doing some online research on Automotive forums and came to the conclusion that this issue was because of a vacuum leak related to the elbow on the hose assembly for the pcv valve. The rubber material dry rots and cracks causing problems. I had the computer scanned and had four codes which were P0171, P0174, P1131, and P1151. After reading posts related to these codes I decided to invest in a new pcv valve and hose assembly and change the valve and hose myself. I also was able to watch a video on how to replace these parts without removing the upper intake. It was a difficult, and time consuming ordeal to replace these parts, but if your patient, the results are excellent. The car starts easier and there is no longer any rough idle. So far the check engine light hasn't come back on yet ether. The parts were a little over $38.00 with tax from our local Ford Garage. The money I saved from doing it myself was also a huge bonus. This may not be the cure for your vehicle problem, but it was the fix for my car. Also, a "HUGE" THANK YOU to the Automotive Forums website for the endless amount of information I find available on this site. It has save me possibly thousands of dollars in repair costs.
Here is the video I found for the pcv valve, and hose assembly replacement:
www.youtube.com/watch?v=ujbfRUCt5uA