@susie79: Are you using any specific apps when the issue occurs? Do you have a custom Theme enabled? Can you try switching off the phone, then press the Power and Volume Up keys together until the Samsung logo appears on screen. Release the buttons at this stage, and you should see a Recovery Mode screen. Using the Volume Down button, scroll to Wipe Cache Partition, and hit the Power button to select it. Select 'Yes', then scroll to Reboot System Now on the Recovery Menu, and tap the Power button. Let me know if this helps.