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'm 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.

You have a vacuum leak at one of the PCV hose elbows. Examine the complete PCV system, there are hard lines and rubber elbows, the rubber elbows rot out and cause your problem. There's a really hard one to get at the back of the block between the engine and firewall.