I started experiencing this recently as well. After updating video drivers, deleting and reinstalling the SL viewere I found the same. I have Firestorm installed too, I must test this but also noticed a slow and steady drop in fps on firestorm.
Opening a blank chat window and leaving it open my FPS jumps way up then as soon as I click on local chat or an IM I've had open for a while, drops back down. Frustrating!