Instability of non-commutative SYM theories at finite temperature

We extend our previous work on the quasi-particle excitations in N = 4 non- commutative U(1) Yang-Mills theory at finite temperature. We show that abov e some critical temperature there is a tachyon in the spectrum of excitatio ns. It is a collective transverse photon mode polarized in the non-commutat ive plane. Thus the theory seems to undergo a phase transition at high temp erature. Furthermore we find that the group velocity of quasi-particles gen erically exceeds the speed of light at low momentum.