I recently was editing a post.aspx page that would crash whenever switching to Advanced Mode to edit the code.
I found a fix from http://morshemesh.blogspot.com/2011/08/how-to-fix-sharepoint-designer-crash.html that was pretty simple.
Go to File > Options > General > Page Editor Options > IntelliSense > Remove check from HTML, ASP.NET from both Auto Popup and Auto Insert.
That’s it. Now I can used Advanced Mode to edit without crashes using SharePoint Designer 2013.
Update: Another trick that works is to check out the file in Designer and then right click to edit the file in advanced mode right from the file browser.