We occassionally see behavior like this from George. I agree that he's not attacking us, but he seems to want to play. The way he gets his sister (littermate) to play is he runs up to her, crashes into her, grabs her tail in his teeth (or her ear, or whatever is available). This is perfectly appropriate behavior between dogs, but not great, as you know, with people.
What works best with George, besides tethering, as you are already doing, is we put him through several days of what I call "boot camp." All this means is that before he eats, plays, gets a treat, walks in the door, in other words, before he does ANYTHING he must sit. Sometimes he gets a treat for sitting, but at meal time he gets his food. When he really needs a wake up call, I make him sit and feed him his kibble out of my hand very slowly. I will then put a few pieces of kibble into his bowl and stand over him while he eats. I do all this very calmly, no loud voices or any tension, but firmly. Sometimes I put the kibble in my hand INTO his bowl and hold my hand there. Sometimes I will tell him to leave his food and make him sit while I stand over him and talk pleasantly to him. When I'm satisfied that I have his attention and he is settled, I give him the rest of his food. It sounds like a strange ritual, but for George, it works! I do this for a couple of days whenever I see him getting a little pushy and bossy with me. It gives us both a lesson. It especially reminds me that I am the steward of these fine animals and sometimes I have to really concentrate on how I interact with them. They are so sweet, but sometimes when I let down my guard it's "give 'em an inch, and they take a mile!" Then we go back to "boot camp!"
I also agree with everyone here that lots of exercise is very important, too. If it helps at all I can tell you that my two doods seemed to really start pushing the limits a lot more at around 9 or 10 months. But they seemed to calm down again pretty quickly. In fact, today they are 1 year old! And they are mostly well behaved. Sorry this is so long, but I know how frustrating this can be and this has really worked for us!