  We investigate a puzzle in the recent thermodynamics of scalar-tensor
gravity, in which general relativity is a zero-temperature state of equilibrium
and scalar-tensor gravity is the non-equilibrium configuration of an effective
dissipative fluid. A stealth solution of Brans-Dicke gravity with constant
positive temperature is shown to be analogous to a metastable state for the
effective fluid and to suffer from an instability. The stability analysis
employs a version of the Bardeen-Ellis-Bruni-Hwang gauge-invariant formalism
for cosmological perturbations adapted to modified gravity. The metastable
state is destroyed by tensor perturbations.
