Weird electrical fault

Hi every now and then when driving or parked i get an odd thing happen, its like all electrical power is cut for just a brief second.

The speedo will drop to zero, the stereo will go off and all lights go off, every electrical item looses power for a split second....

Anyone have any ideas as to were to start to look for this problem?
Is this a common to discoveries fault?

I have searched and cant find anything to help..

Not at all sure were to start.

Many thanks for any advice as this is driving me mad.

I'd start by checking all earth and battery connections and as far as possible fusebox connections.

And the back of the ignition switch.