loosing power

My brother has a '92 aerostar extended van with a 4.0 4-speed auto. When he is driving at highway speeds in 4th everything is great. The van will go with no trouble, but when he hits a hill and he pushes it a bit and it shifts down a gear the trouble starts. He says its like the engine has no power and continues to loose speed. I asked him if it seemed like it couldn't breath and he said maybe. He is going to check his air cleaner but I was wondering if it might be a plugged cat. converter. It runs fine but just when a load comes on the engine it has no power. The engine will not pull to hold or pickup and speed.