I have been having the same issue lately. I went to the trouble of resetting to factory defaults and then formatting the memory and reloading my books.
I am beginning to believe it may be the ePub I am reading at the moment. It is a commercial ePub that I used Sigil to change some of the formatting on and removed unused styles (of which there were many). I will try to run the ePub checking to see if I have any problems. But I will be following this thread to see if you find anything.