  Necessary and sufficient conditions are given for the Palais-Smale Condition
C to hold for the Yang-Mills functional for connections that are invariant
under a Lie group action on the manifold with orbits of codimension less than
or equal to three. As an application the mountain pass lemma is used to give a
simple proof of Wang's theorem on the existence of irreducible
non-(anti-)selfdual instantons on S^2 x S^2 .
