  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
above some critical temperature there is a tachyon in the spectrum of
excitations. It is a collective transverse photon mode polarized in the
non-commutative plane. Thus the theory seems to undergo a phase transition at
high temperature. Furthermore we find that the group velocity of
quasi-particles generically exceeds the speed of light at low momentum.
