  Using Monte Carlo simulations, we compute the spin stiffness of a site-random
3d Heisenberg model with competing ferromagnetic and antiferromagnetic
interactions. Our results for the pure limit yield values of the the critical
temperature and the critical exponent $\nu$ in excellent agreement with
previous high precision studies. In the disordered case, a mixed "chiral" phase
is found which may be in the same universality class as 3d Heisenberg spin
glasses.
