I noticed the same thing on my cleric in the fiery pits. It's like if I don't kill then fast enough they buff up or get enraged and become almost impossible to kill.
It's cool and everything but won't it make the AD exchange skyrocket? Might be actually cheaper to buy with real money? And that's probably what the developers are going for
PvP PvE it's pretty bad anywhere. It happened to me a lot when casting hallowed ground as cleric. I thought that was the problem but it seems it is not as the game has frozen on me on several times in a dungeon and I didn't cast anything.