Checkout page has incorrect formatting and freezes

Hello~
I just rebuilt our checkout to work with both PayPal Pro and with our FedEx account. I noticed that the checkout page wouldn't display correctly, and then when I tested the cart, found that it's not working.

1. When I click 'checkout' in the cart, it won't forward to the checkout page even though I've set the button to forward.
2. I can type the address to the checkout page and it shows the items in my cart, but it's displaying strangely with what looks like an i-frame of the shopping cart below the billing and shipping address, but it's not aligned and looks terrible.
3. Once I enter the addresses and click continue, the progress bar pops up but the page gets hung up. Also, the 'cart' from the i-frame moves to the top of the page and the bottom i-frame changes to the page asking for my credit card information.

That's as far as I can get because the page freezes.

I updated to cart 6 today and updated all the extensions to the most current.

Thanks in advance for your help.