This isn't normal but nobody knows what it is?

I drive a 2002 Ford Foucus SE with 2.0 spi. My car will downshift randomly but only when the ac is on. When the ac is on my interior and exterior ;ights flicker, and my engiene idles at about 450-500 rpm. I have replaced the idle air control valve,ecm,ac compressor,alternator and battery. I am very unsure of what this could be. I took the car to ford for a full point inspection and to have the car scanned. They said nothing showed.

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