We present a theoretical and experimental study of the oscillatory propagation of lean premixed methane-air flames in tubes of low thermal conductivity. The effect is observed experimentally near flashback conditions when the flame velocity relative to the wall is much smaller than the planar burning velocity. The theoretical and numerical analysis is based on a simplified constant-density model. It is shown that the principal mechanism of oscillation results from a thermal flame-wall interaction, and that there can be multiple auto-oscillatory states for the same set of parameters.