Known issue, can be various things. Low polycount, smoothgroups, shadow bias. Usualy only happens when camera and the light are at 90 degr. from one another.
If it's smoothgroups select all faces and turn on autosmooth. Low polycount use subdivide.
And it might not be solveable at all. Shadow mapping can always give some flickering in shadows. Other option is to get rid of the first frame and copy the second.
