Hello,
I observed the same problems with my 505, when I used LRF-books converted with a recent version of calibre. But I don't remember which number it was. I also have PRS+ installed.
Now I use Book Designer to make LRFs and I haven't any problems since then. When you get used to this programm it's great, great, great

, although sometimes a bit weird...
I save both lrf and html0 (that's the bookdesigner filetype) and I am able to edit that file in case of typos etc. and convert again in LRF. I love the footer (with chapter info) created with BD.