  We consider a massless, minimally coupled scalar with a quartic
self-interaction which is released in Bunch-Davies vacuum in locally de Sitter
background of an inflating universe. It was shown, in this system, that quantum
effects can induce a temporary phase of super-acceleration causing a violation
of the Weak Energy Condition on cosmological scales. In this paper we
investigate the system's stability by studying the behavior of linearized
perturbations in the quantum-corrected effective field equation at one and
two-loop order. We show that the time dependence we infer from the
quantum-corrected mode function is in perfect agreement with the system
developing a positive mass squared. The maximum induced mass remains
perturbatively small and it does not go tachyonic. Thus, the system is stable.
