  We investigate a two-component mixture of resonantly interacting Fermi gases
as a function of the ratio \kappa of the heavy to the light mass of the two
species. The diffusion Monte Carlo method is used to calculate the ground-state
energy and the pair correlation function starting from two different guiding
wave functions, which describe respectively the superfluid and the normal state
of the gas. Results show that the mixture is stable and superfluid for mass
ratios smaller than the critical value \kappa_c = 13 +/- 1. For larger values
of \kappa simulations utilizing the wave function of the normal state are
unstable towards cluster formation. The relevant cluster states driving the
instability appear to be formed by one light particle and two or more heavy
particles within distances on the order of the range of the interatomic
potential. The small overlap between the wave function of the trimer bound
state and the guiding wave function used to describe the superfluid state
produces the unphysical stability of the superfluid gas above \kappa_c.
