The aim of the paper is to study the problem where Ω is a open domain of R N with uniformly C r boundary (N ≥ 2, r ≥ 1), Γ = ∂Ω, (Γ 0 , Γ 1 ) is a relatively open partition of Γ with Γ 0 (but not Γ 1 ) possibly empty. Here div Γ and ∇ Γ denote the Riemannian divergence and gradient operators on Γ, ν is the outward normal to Ω, the coefficients µ, σ, δ, κ, ρ are suitably regular functions on Γ 1 with ρ, σ and µ uniformly positive, d is a suitably regular function in Ω and c is a positive constant.In this paper we first study well-posedness in the natural energy space and give regularity results. Hence we study asymptotic stability for solutions when Ω is bounded, Γ 1 is connected, r = 2, ρ is constant and κ, δ, d ≥ 0.