Lincoln/Mercury Repair: 2000 Mercury Sable Idle Problems, mass air flow sensor, mass air flow

Question
Alright, here is my problem.  Back in August 2004, I left my car sitting out (mild temperatures, 40-70 degrees per day) for approximately a week.  When I went to start it up, the car would not reach an idle point and stalled out.  Had it taken to the dealer, at which point they replaced the IAC, and I installed a new battery at an AutoZone.  The idle at this point was still fairly rough, and at occasional intervals it would drop to around 500-600 RPM and really cause the pedal/dash/steering wheel/etc. to vibrate violently.

Flash forward to 2 weeks ago.  The idle problem appears to be more prevalent now than it had been in the past.  The 2000 Sable idles around 550-600 RPM after the engine is warm, and will noticeably drop down to the 500 range, causing the car to really shake.  A passenger in the car can easily feel the vibrations caused by the RPM drops, which feels like the car is going to stall out (though it has not).  The normal idle is fairly rough, and I can feel mild vibrations in the steering wheel and pedals even when the car is NOT experiencing the RPM drop.

So I took the car to a dealer, in which they replaced the Mass Air Flow sensor, which has still not corrected the problem.  I took it BACK to the dealer yesterday (it was raining), and neither the mechanic, shop foreman, or myself could re-create the problem.  Flash forward to today, and I was able to cause the RPM drops and rough idle again after about 9 miles of driving, and sitting either parked or in drive with the brakes applied.  I even had one RPM drop which actually went BELOW 500 RPM and felt like the car was going to stall out.

Summary.  I've had the IAC and MAF sensor replaced.  The battery is new, and the dealer can not find anything wrong with the car in diagnostics.  At this point, I am tempted to see if I can create the RPM drops, and just drive over to the dealer when I'm 100% sure I can have it done.  But I don't know what they could suggest since they diagnostically can't see anything wrong.  I did suggest a possible vacuum hose problem, but they did not respond to this either way.

Help?

Answer
hello
this is a vac line or a idile controll senser rember if the car is runing good when they hook it up to diagnosyiac test it will not show the bad senser . i belive vac lines or idile controll senser . good luck to u and please rate my serivice pete