You are using an external monitor? Just asking because a resolution of 1920 x 1080 seems a little bit uncommon for a laptop. That resolution is also asking a lot from a GeForce GT 540M (in games at least, desktop environment shouldn't matter).
If you had freezes before updating, too. It's most likely GPU overheating. Still got warranty for your laptop? If not open up and clean it yourself. If you do, sent it in.