funny enough i've encountered the same problem. lucky for me that i can compare my device with the one of my wife. we have the same DR800SG, didn't change the sd-card, have exactly the same books on it and the same beta software. only by looking what device currently is reading what book, is the only way to find out who's reader it is and of course by serial number. interesting the the problem only occur at my device and not at the other one. my device has the same problems as jjansen. first i took of all the beta stuff and kept the device with the original standard software. then it worked great. slowly i start to upgrade with the beta versions and currently my device is just a fraction slower as the other one. for now acceptable. if it continue to occur, i'm going to swap sd-card.