We study in a strip of R 2 a combustion model of flame propagation with stepwise temperature kinetics and zero-order reaction, characterized by two free interfaces, respectively the ignition and the trailing fronts. The latter interface presents an additional difficulty because the non-degeneracy condition is not met. We turn the system to a fully nonlinear problem which is thoroughly investigated. When the width of the strip is sufficiently large, we prove the existence of a critical value Lec of the Lewis number Le, such that the one-dimensional, planar, solution is unstable for 0 < Le < Lec. Some numerical simulations confirm the analysis.2000 Mathematics Subject Classification. Primary: 35R35; Secondary: 35B35, 35K50, 80A25.