My DOCX export turns into pages of random characters partway through the book, and Word keeps freezing on it. What is going on?

Fixed in v1.8.4 — and your book heals itself. The cause was surprisingly specific: pictures that were BOLDED in the original Word document (or placed mid-sentence) reached the exporter in a form it didn't recognize, so the image's raw data spilled into the document as an enormous run of text — those "random characters" WERE your picture's data, and their sheer size is also what made Word stall with "not responding". Since v1.8.4, bolded and mid-sentence images embed as real pictures; books that already imported such images are repaired automatically the next time you export (no re-import needed); and a safety net guarantees image data can never appear as text in an export again. Just update and export to DOCX again.