  We explore a dark energy model with a ghost scalar field in the context of
the runaway dilaton scenario in low-energy effective string theory. We address
the problem of vacuum stability by implementing higher-order derivative terms
and show that a cosmologically viable model of ``phantomized'' dark energy can
be constructed without violating the stability of quantum fluctuations. We also
analytically derive the condition under which cosmological scaling solutions
exist starting from a general Lagrangian including the phantom type scalar
field. We apply this method to the case where the dilaton is coupled to
non-relativistic dark matter and find that the system tends to become quantum
mechanically unstable when a constant coupling is always present. Nevertheless,
it is possible to obtain a viable cosmological solution in which the energy
density of the dilaton eventually approaches the present value of dark energy
provided that the coupling rapidly grows during the transition to the scalar
field dominated era.
