PRS-300 Have to reset often

[dura]

I've been enjoying my blue PRS300 for almost a year now, but recently I've got problems.
Half of the times the reader won't start and I've to perform a soft reset.
It started more or less at the same time I decided to convert my books from LRF to epub (in the light of an upcoming tablet buy).
I've tried everything: reformat from the reader, reformat form PC, uninstall sony reader and only use Calibre, Not use Calibre but only the reader software, throw away the database files so they get recreated, reconvert the sourcefiles, use an external power adapter etc. etc.
No success, it still starts up al right, or it doesn't, and I've run out of ideas?
Is it defect? But it works perfectly okay once started up.
Or are the epubfiles in some way corrupted? They look fine..

Can anyone provide new insights, or do I have to sent it back or learn to live with it?