Automatic data erased

Sir/Madam, i am using iPhone 3gs , there is automatic erase of the contacts any time. Also the screen gets hang up & some times it gives flickering. Even the applications run quite slow, i.e. the camara shutter.

MR DAVID Sir, Many thanks : i would definitely follow process to full restore and updates of the firmware : i will be thankful if you can please guide me to check the liquid damage. this phone is new one , about 3 months and not exposed to rain or any kind of water.