For public domain classics, try downloading the same title from the public domain section of feedbooks.com
in EPUB format.
I have found that Feedbooks has the best formatted public domain material.
Be careful when you do a search on feedbooks, because often it will list non-free versions first, even though, if you scroll down a little, you can find a public doman version available for free download. (if you do an 'advanced search' you have the option to only search within the public domain section, and can avoid this)
The reason you are running into problems is that Calibre does a really crappy job of converting plain text documents.
The reason that Calibre has problems, is because many older books in plain text format include hard-carriage-returns at the end of each line, and Calibre can't re-flow them properly. Calibre likes plain text to include carriage returns or line-ends only at the end of a paragraph, which is what is needed for free-flowing text for an eReader like the Sony.
When you push one of these text files through Calibre, it either wants to leave things as they are, which is pretty lame because then the book looks like crap on your Sony, or, even worse, Calibre will take out ALL the line-ends completely, leaving you with your 350 page book reduced to a single run-on paragraph (really, really, STUPID).
It's usually easier to just download properly formated books from sites like feedbooks, but if you do want to try to clean up any books you can only find in plain text format, there is a PC program called E-Book Tidy which you should be able to find on the web.
E-Book Tidy will let you remove unwanted line-ends (Join Lines) while leaving proper line-ends at the end of each paragraph, so the text will look good on your Sony.