I think u have a problem with ur software caz that's not normal...try to format ur mobile by tiping this code:*#7370# and see if the problem presists...
N.B.:remember to backup all ur phone memory data,ex:contacts,inbox,media,etc.... because ur phone memory will be formatted and all the settings will be rendered to default.
or,update ur firmware.and if it's up to date reinstall it.....