Cheers Kovid.
Hope this is not too late?
I also am getting random freezes/crashes using the excellent built-in link snippet <a and as of today am consistently getting an immediate crash at one site in the epub book.
If I take the file out of the book as a separate 'book' the crash does not occur.
Running in debug mode simply yields 'Segmentation fault (core dumped)'.
I'm running Calibre on Ubuntu 16.04 64-bit with no other (noticeable) programs running.
How can I analyse the fault?