CPU Performance – Why cat /proc/cpuinfo Shows Different CPU MHz and Model Speed

There's two entries for each core, because Nehalem (Xeon 55## series) supports two hardware threads per core, which to Linux appears as two processors. The CPU frequency varies because each core can be independently clocked; going by the physical id value the two you've posted appear to be different cores.

Edit: Yes, frequency scaling is why the chips have different speeds. Linux's frequency scaling is pretty good these days days, so there's no harm in leaving it enabled, and it'll save you power (both directly and from reduced cooling costs).

Have you tried booting a different kernel/distribution? Perhaps it's just a kernel bug. Try a fedora LiveCD, that should be suitably different from your Ubuntu.