When people get synced down to level 50 in those two dungeons, they get synced to roughly iLevel 120. When the content you're running was made, it was made for ~iLv50. Because of that huge descrepancy, stuff dies way faster than it did when the content was fresh.

You are running content that is 6 years old at this point. It's bound to go down fast. Just deal with it. There are only two 8-man main story dungeons, and after that, they learned that it wasn't a good idea, and they never did it again. Just get it over with and continue on with the game.