First, have you done a battery pull recently? Battery Pull: Leave your device on, pull the battery for 20 seconds, put it back in and wait for your device to boot up.
Second, have you monitored what is running in the background? There should be only 5 apps running in the background, as part of the BB OS, those are: Home Screen, Messenger, Browser, Call Log, and Messages. You can find out what is running in the background by holding down the Menu button for about 2 seconds, then let go. You can scroll left and right to look at the entire list, if there are any besides these you are not closing it properly or it's an app you want to run in the background.
