Some added insights for those dealing with this issue (conversion of large and very complicated books):
I have the NIV Study Bible by Zondervan (41.6 MB as a mobi file that has been moved into Calibre from a purchase in Kindle for PC software) and after doing research online, I installed the 64 bit version of Calibre on my best quad core PC with 8 GB of memory and set it in motion 47 hours ago converting from mobi to epub (for personal use on two iPads and two iPhones) and I just got home to find it finally done! I had tried the 32 bit version of Calibre a few times with various large Study Bibles and I always got errors before. Three Kindle Keyboards are the main reading devices in my home, but iPads and iPhones are growing in use and having things on them (in iBooks) is handy (not a fan of the Kindle software, just the hardware).
I also have the ESV Study Bible (47.8 MB), NIV Faith in Action Study Bible (18.8 MB), and the Life Application Study Bible NLT (22.8 MB) so now I will try them to see if they will convert too. When the 64 bit version was running, the Calibre process was using nearly 6 GB of the 8 GB of my total memory and 25% of the CPU! But the 64 bit version and the memory beyond 4 GB appears to be the key to prevent the errors.
Iíve been poking around mobileread for years and this problem finally got me to sign up so that is a good thing. A nice site that has provided me several important bits of information over the years.
Thank you for your efforts.