Comment 9 for bug 400413

I believe I found the root cause:

$ cpufreq-info |grep policy
  current policy: frequency should be within 933 MHz and 933 MHz.

So the frequency governor cannot select a faster speed than 933 MHz even tough the CPUs support 1.6 GHz.

I changed /etc/default/cpufrequtils to:

MAX_SPEED="1600000"

and have not seen the issue since.

A better fix would be to have /etc/init.d/cpufrequtils default to the max value if MAX_SPEED = 0.