Use the electronic odometer trick to monitor battery voltage. I'm thinking you're having alternator problems and your voltage will drop from 14 to 12 when the AC is on. Low voltage can cause all sorts of issues with sensors, and might be causing your downshift.
To use this trick is easy: key off, press and hold the odo reset button. Turn key on, continue to hold button until the odometer reads [test], release. Now the odometer is in test mode, and will stay in test mode until you turn the key off. Each time you press and release the odo reset button, you'll scroll through one of 30 test functions. The one you're looking for reads like this [bat 12.1] or whatever your voltage should be. Voltmeter readings at the battery post will be about .2v higher than this reading. Now start the car and you should be able to monitor your alternator voltage once the car is running. It should be between 13 and 15v. Higher than 15, or lower than 13 means you have alternator problems.
I highly suggest that you find a local rebuilder in the yellow pages. Not many of us have had luck with parts store rebuilds. Local rebuilders typically use better quality and more expensive voltage regulators because they don't want the warranty returns. Parts chains don't care. They don't make money off individual customers- they make it off thousands sold. Local rebuilders are also cheaper, and some will only replace broken parts which can save you big bucks since the pulley on our alternators are $85. I paid $80 for my alternator to be rebuilt. At a parts store, a cheap rebuild was $165, lifetime warranty was $185, and I think a new Motorcraft is like $225ish. These places don't usually advertise online, or in paid ads in the YP, but they will be in the YP business section. Their bread and butter is local shops- who then turn around and charge the customer parts store price for the part.
LOL you gotta love/hate the auto business. Tote the note used car dealers are even worse- I swear.
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