You know, I feel the same thing with my Heckler, and can swear it's the swing arm flexing. When I look down at it though, it looks rock solid. I'm fairly sure most of the flex we feel comes from the wheels, tires, and possible at the hub even if the QR is really tight.
One way I check is to put the rear triangle against a wall and lightly push sideways on the front. My old superlight definitely had flex at the pivot, but my Heckler is perfectly stiff. Then try pushing with your knee against the edge of the rear wheel (softly), while pushing the other way on the front of the bike. You'll probably see there is some moderate flex in the wheel.