Help with lights flickering at weird regular interval
I had strange flickering in the past (but not quite with the strange periodicity you are), and this is how I found the device(s) that was dirtying up the power for my whole house:


Go to the breaker and shutoff all but one of the flickering circuits


Did the flickering stop? 

if yes:


One by one turn each circuit back on until the flickering returns
Once flicking returns you now know which circuit contains the device causing the problem (the last one you turned on). One by one, unplug/unscrew each device/bulb from the circuit until the flickering stops


if no:


Turn off the circuit, and switch on a different one that you had noticed flickering on


If the flickering is gone, it was due to a device in your home, proceed to step 3 to isolate the device.
If you are still experiencing flickering, it is likely due to a problem your electrical carriers equipment.  Call them and let them know you've isolated the problem to something on their end.