Unfortunately unless it's current content you are going to find the old content is going to be a cakewalk until you get up to the current content, because everyone is going to be over geared and can pull the whole dungeon because it's quicker. Unless you do content when it is released, where tanks suddenly realise they cant pull the whole dungeon you are always going to have this problem. The only other choice is to get into the high end game content where the difficulty jumps.