  We further analyze a class of recently studied metastable string vacua
obtained by wrapping D5-branes and anti-D5-branes over rigid homologous S^2's
of a non-compact Calabi-Yau threefold. The large N dual description is
characterized by a potential for the glueball fields which is determined by an
auxiliary matrix model. The higher order corrections to this potential produce
a suprisingly rich phase structure. In particular, at sufficiently large 't
Hooft coupling the metastable vacua present at weak coupling cease to exist.
This instability can already be seen by an open string two loop contribution to
the glueball potential. The glueball potential also lifts some of the
degeneracy in the vacua characterized by the phases of the glueball fields.
This generates an exactly computable non-vanishing axion potential at large N.
