I have a 2006 Ford Taurus with ~70195 miles.. It's been to the shop 4 or 5 times now and it runs fine when I get it back. Then after about an hour or so of driving time the check engine light comes on and the RPM falls when I let my foot off the gas and the vehicle dies. At first we assumed it was the Low Idle Valve or whatever and we got that replaced for $100. And also at the time my AC wasn't blowing cold air so we figured it need a partial A/C charge or whatever. So we did that too. We have no idea what the problem is and so I decided to come look here on the internet and see if anyone else has had these problems. Any ideas as to what the cause is?

 

There are many things that could cause your engine to run rough, stall etc.,. Some auto parts places will check your vehicle's computer for fault codes at no charge.
Among them is Autozone. Since the check engine light is on, there should be a code(s) that will direct us to the problem area. That code(s) would be a good place to start.
You need to be a little more specific regarding the A/C system.
Does the blower work at all?
Does it start out cold then get warm? Be as specific as you can . Did whoever added the refrigerant to the A/C system use any gauges to make sure they didn't overcharge the system. In the process of adding refrigerant could moisture have been injected into the system?

 

The vehicle will run fine for the first hour or so of driving. It will blow cold air. And then once the engine light comes on the cold air stops and it starts to blow warm air. Then whenever I let my foot off the gas there is a drastic change in RPM and when I come up to a stop or slow down below 25-30mph I have to shift it into neutral and keep the RPM's from falling to zero and killing the car.

 

Right now it seems that the next thing to do is get the codes.

 

i had the exact same problem in my taurus. the shop figured out that a fuse was blown due to the AC compressor drawing too much power. right after the shop fixed that i was able to idle, although i still idle rougher than normal, its exponentially better than it was. right after the shop "fixed" the problem by disconnecting my ac the ac clutch pulley stopped spinning. i had to get a smaller belt and reroute to bypass the ac. so my suggestion to you is disconnect the ac compressor, look for a blown fuse and change it, and watch to make sure the ac clutch doesn't seize up on you.