You can get many old versions at the
Sourceforge project page, I don't know if that will fix your problem, though. Also, read the changelogs before you downgrade, I remember there being a change not too long ago that breaks downgrades past that version. My personal advice would be to stay on 0.7.52- downgrades, while mostly possible, aren't officially supported for Calibre.
You're definitely not having a connection problem, then. You're saying that some books give errors when you try to transfer them- is there anything that distinguishes those books from the ones that transfer OK? Is the MOBI format available for them or does Calibre have to do an autoconvert before sending them?