I am a fan of the Nexus drivetrain. Just had to say that. Anyways, the symptoms appeared when you removed and reinstalled the wheel. There are a couple of washers on each side of the axle that aid in spacing and keep the wheel in place. Two of these washers are called anti-rotation washers. They have tabs that fit into the channel of the frame and are square in the middle so they lock into the hub. My guess is that these are not installed properly. Under torque (high gear out of seat pedaling) the hub is shifting in the rear dropouts. The Nexus are sensitive to misalignment. The washers have to be inserted properly. The tabs must fit into the frame. Then the bolts have to be on tight, but not super-tight. The tabs must fit into the frame. Good luck!
