You could be starting to experience balance problems. Put the bike on a rack, put it in a high gear, and spin it up to a high speed, and see if the whole bike starts to move in time with the rotation of the wheel. If one side of the wheel is heavy, the bike will tend to move forward when that bit moves backwards, and vice versa, as the wheel tries to keep its center of mass in the same position.

You may be able to trim this by messing with spoke reflectors, trimming them in or out, or using lighter/heavier reflectors to counter the valve stem weight.