How many books are on your SD card?
If you try to load a poorly formatted book the reader will quit processing books into it's database. This could be your problem. When loading boos from questionable sources only load 10, 20, 100 what your comfortable with removing then loading one at a time to find problem book. Some more savvy can look at data base to find last book loaded which the book causing reader to stop working.
How did you update your reader's firmware to 1.9.14? Only update with software update that loads during sync with Kobo servers. For Readers bought in Australia it is 2 step precess first to 1.9.14 then another sync to upload and install 2.1.5 Version 2.1.5 is much better and trouble free for me.
Are you using book shelves? How many shelves? How many books on each shelf? How many books on shelves total?
The last book I loaded was one that I got via wifi from the Kobo store, when I was downloading that the update started and finished and from there its gone downhill...
I am using book shelves... I think around 6-8 and no more than 10 on each...