Common problem with 6500 slide. Usually your memory card is either wrongly fitted or loose. Consequently any movement of the phone means that files on it are no longer available and the phone resets to defaults for wallpaper etc. Sometimes if your back cover gets squashed it shorts on the memory card/card holder. People sometimes put a little tape over the card holder once the card is inserted to prevent any shorting out on the rear cover..
You could also ask here (mobile phone section of Digital Spy forums);
http://www.digitalspy.co.uk/forums/forumdispla y.php?f=1