I've dealt with heating problems with PCs for years and, although I haven't received my 360 yet, the freezings sound consistent with heating issues.

Why Perfect Dark Zero freezes up more than other games can be explained.   Some games require more processing and load on the cores.  This, in turn, requires more power and produces more heat.  This is true with PC games as well.  Some games tax the processors and heat them up much more than other games.     PD seems to be a game that taxes the processors and produces more heat.