Crashing on Epubs generated from pandoc

I've been converting some markdown files to epubs using pandoc. They are pretty simple files. Calibre "chokes" on these when importing and I have to kill the process. They DO end up being imported... it's just messy.

AI suggested using this switch:

 -f markdown+ascii_identifiers 
and possibly this one:
 -t epub3

What's weird is these seemed to work for a couple of epubs, but now I'm back to seeing crashes.

Any ideas?