  We analyze the dynamical stability of gap solitons formed in a quasi
one-dimensional Bose-Einstein condensate in an optical lattice. Using two
different numerical methods we show that, under realistic assumptions for
experimental parameters, a gap soliton is stable only in a truly
one-dimensional situation. In two and three dimensions resonant transverse
excitations lead to dynamical instability. The time scale of the decay is
numerically calculated and shown to be large compared to the characteristic
time scale of solitons for realistic physical parameters.
