I had this on mine for a long time, first use system restore in "safe mode" and it may cure it for a while.
The problem seemed to occur every time Norton Av updated.
Having read the notes on the present AV that I got with BB, it seems there can be a conflict set up with firewalls. Make sure there is only one switched on.
Ah just notced the safe mode option fails - makes it worse than mine then, the rest of the sympyoms match though