I get that regularly(2-3 times an hour at least TT_TT), and I know it's my internet connection- are you using wireless?
If you're using your laptop's built-in wireless it may not be very good, you could try with a cable from your modem and see if that fixes it.
Of course, if you're cabled already I don't know what it could be.
Edit, sorry, I thought you meant you were playing on a laptop, but I reread and that's not the computer you're having problems with is it?
Thus, my advice is invalid unless you're on wireless, in which case you may as well try.
But Norton is an infamous nuisance, you're probably going to want it of your computer eventually for one reason or another. It could easily be that.