This is usually caused by one of or a few different issues, all of which are nearly impossible to troubleshoot remotely.
- You did not include certain required layout assets.
- Improper markup
- Invalid web part references.
- Invalid system layout id references.
A couple of techniques that can be used to diagnose this further.
Start with the default template in sharepoint designer and keep changing slowly until you reach your final layout.
Wait for the page to timeout or view the console in developer tools to see if there is any details.