Louis, where did you find the "Free Shipping - Free Rules"? Is it this one (
https://www.zen-cart.com/downloads.php?do=file&id=428)?
A couple of things to check when OPC goes into a shipping-request loop like that:
1) Check your browser's "Console Log". Press F12 on any of the major (FireFox, Chrome, IE) browsers and then choose the "Console" pane. That is used by OPC to report any jQuery/javascript related issues. Go to the checkout process; are there any errors reported?
2) Generate a OPC log (I'll send you a PM with my direct email since those logs can get to be pretty big). Turn on the Configuration->One Page Checkout->Enable Debug and make sure that all the /logs/myDEBUG-one_page_checkout*.log files are erased so you're starting fresh. Add a product to your cart, go to checkout, select the problematic shipping method and let the issue occur. At that point, grab a copy of the log file generated and send that to me.