The dynamical behavior of freely-propagating premixed gas flames in tubes is studied experimentally in the high-Lewis number (Le) regime using a mixture of butane and oxygen diluted with helium (Le approximate to 4.0). Steadily-propagating, stable flames develop traveling wave and spinning instabilities as the stoichiometry of the mixture approaches the lean flammability limit These instabilities occur spontaneously and form target patterns and rotating spiral waves, bearing remarkable similarity with patterns observed in excitable media. Additional experiments are conducted on burner-stabilized flames and indicate that these instabilities also occur in three common air-diluted mixtures with smaller Le＇s than their helium-diluted counterparts. Specifically, the instabilities are observed in stoichiometric and lean mixtures of butane-air (Le approximate to 2.1), propane-air (Le approximate to 1.9), and stoichiometric and rich mixtures of methane-air (Le approximate to 1.1) at sufficiently low mixture flow rates. Both experiments confirm model predictions that increasing conductive loss lowers the critical Le required for onset of instability.