TD, your description is usually caused by a badly terminated Placeholder's code: everything will render until it hits it and all the styling and features go out the window.

To test a site or page, I would drag the Placeholder)(s) onto the Pasteboard and Preview.
If you understand the browser's Dev Tools (F12) then you can refresh the page and watch the Network transfers and the error messages in its Console.

The other approach is divide and conquer: half the number of pages, layers or objects until it works. You then half again until you find the fault.

By far the best is to develop a bit at a time and test every stage, saving versions as you go along.
Pick a good template first and dissect it until you understand what it does and, importantly, how.
Start simple.

Acorn