| The problem could be with your preventive maintenance program. I have a 2000 Taurus, with the 3.0L Duratec Engine, and AX4N transmission. I hope you have been dropping/cleaning the pan and changing the filter every 35,000 miles (which should be no more than 30,000 miles), not just using a fluid exchange machine. These transmissions generate quite a bit of crud between changes. I suspect varnish is developing on the servos. I only use Amsoil Universal Synthetic ATF or Mobil 1 Multi-Vehicle Synthetic ATF in my AX4N and change the fluid and filter every 25,000 miles. It's best to do a complete drain, including the torque converter. My Taurus gets driven hard in the Texas heat and shifts flawlessly. Automatic transaxles are electro-mechanical marvels that are more involved than engines these days. They are the weak link in vehicles and need to be carefully maintained. I recommend you only use a full synthetic Mercon V rated ATF. They will keep your transmission much cleaner, cooler, and shifting like new (especially in extreme temperatures) for years of hassle-free driving. The flow characteristics of full synthetic ATFs are vastly superior to regular Mercon V and I would change your fluid again, this time using Mobil 1 Multi-Vehicle Synthetic ATF ($5.68 per qt. at WalMart). This will more thoroughly clean your transmission and likely eliminate your downshift problem.
Install a B&M transmission drain plug in the pan to make it easier next time to change your ATF. | 
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