I would avoid placing the stab directly in line with the chord line of a symmetrical section wing. This seemed to cause a peculiar pitch instability in level flight, which I think was caused by interaction of the vortex generated by the wing with the horizontal stab. I fixed the problem by giving the wing about 1/2 degree positive incidence, and never again placed the stab on the same line. Had the same problem many years ago with a control line speed model.