I'm going to guess this just started after an update to Denim?
Have you tried a Soft Reset? You won't loose data, hold down the Volume Down and Power buttons until the phone vibrates and reboots (about 10 seconds). It will clear out system memory.
If that does not help you may have to Hard Reset which does cause hair data loss...
I think so, but what will happen if i do a soft reset? I was think to do a soft or hard reset,but i just don't want lose all of my app,i have five heavy game are installed. I ask for another solution without hardreset, thks by the way