If you converted a large doc file to a book and your epub contains only one large .html file with all chapters in it, the file is most likely too big for the RMSDK engine in your Sony.
In that case it might help to import the book to Calibre and to do an epub to epub conversion. (Calibre will automatically break up the file into smaller chunks.)
Alternatively, you can insert Sigil chapter marks and have Sigll split the .html file into several smaller files.
For example, if your chapter headings are enclosed by <h3>...</h3> tags, you could use the following search and replace parameters:
Replace: <hr class="sigilChapterBreak" /><h3>
Then select Split at Chapter Markers from the Edit (F6) menu.
However, you might end up with some chapters that are larger than 260KB, which most older readers can handle.