iBook PDF Files Display Problems

The iBook on my iPad has started acting erratic suddenly. I've NOT upgraded the iOS at all, it's still on its original version, 3.2.1. The problem pertains to the display of pdf files (magazines, books etc); I've got these files on iBook for quite some time already, and they displayed OK until now. The various symptoms are:

- when I tried to open a file, error msg says "Book Format Error - This book cannot be opened because the format is not recognized, or is invalid". It's a pdf file! If I leave the file alone for quite some time, I may be able to open it successfully.
- after I open a file, the words are blurry and cannot focus properly.
- after I open a file, the pages are blank.
- the book icon has no image, just the default beige colour with the book title.
- the book icon (cover) takes on the image of a totally different file, e.g. The Economist magazine will show the National Geographic cover instead.
- sometimes the iBook app will crash.

Note that some files will display OK, but others will encounter the above problems. A file may be OK now and problematic later.

Here's what I tried to fix the problem, but no use:
- restarted iPad.
- ensure that memory is above 100MB (in case it's related to memory; iPad has 256MB in total memory from what I know).
- removed all books/magazines, and re-installed a smaller set. Note that all the files displayed perfectly before.
- removed the iBook app, download on iTunes, and re-install.

Note that my epub format books are OK all these while. Only the pdf books/mags are affected. Please help!