Frustrating A/C problems-not cold-whistle
 

I have a 2005 Ford Freestar. The a/c used to be so cold that it would run you out of the vehicle. A year ago, the rear air quit working. One mechanic told me it was the expansion valve in the rear of the vehicle on the driver's side. The local dealership said this,too. They also said it needed a new compressor, condenser, & orifice tube. I did not want to pay the quoted amount. I took it to a third mechanic. They gave the same diagnosis as the dealership. They made the repairs mid-August 2011. By mid-December, it was no longer blowing cold. I had a 2 year warranty and did not need the a/c during the winter. In February, I took it back. There was a leak and it cost me $40. Then about a month later it quit working, again. I had to replace the a/c manifold hose. It cost around $500. It worked well for about 2 weeks. Then it started to not cool at an idle, but worked somewhat better at highway speeds. I took it back to the mechanic. They didn't like something and replaced the whole system, again. It worked for 3 days. Then wouldn't blow cold, at all. I took it back to them. They had it for a week. They, again, replaced the compressor & condenser. They also replaced the evaporative cooler. (I'm not sure what all they did as I did not have to pay for the repairs because of the warranty). I have it back and it is blowing cold, but is not nearly cold enough. Again, it gets colder at highway speeds, but around town or at an idle, it does not cool the vehicle. There is an intermitent high-pitched whistle from the rear passenger quarter panel (where the expansion valve is located). The fans near the radiator come on as soon as you turn the a/c on. I have an out of town ball game and need to drive the van. Any help on this problem would be great.

We just drove the van downtown to watch the fireworks and the a/c was blowing cold. Outside temp was only in the high 70's.