  We use the canonical energy method of Hollands and Wald to study the
stability properties of asymptotically flat, stationary solutions to a very
general class of theories, consisting of a set of coupled scalar fields and
p-form gauge fields, minimally coupled to gravity. We find that, provided
certain very weak assumptions are made on the coupling coefficients, the
canonical energy method can be extended to this class of theories. In
particular, we construct a quadratic form E on initial data perturbations, with
the properties that E > 0 on all perturbations indicates stability, while E < 0
on some perturbation indicates instability. Furthermore, we show that the
conditions needed for the existence of E allow for a stable definition of
asymptotic flatness. Finally, we extend the proof of the Gubser-Mitra
conjecture, given by Hollands and Wald, to this class of theories. In
particular, this shows that for sufficiently extended, charged black brane
solutions to such theories, thermodynamic instability implies dynamical
instability.
