I have noticed that several flights I have had (especially from SEA for some reason) that when we are beginning to back away from the terminal suddenly everything goes dead, lights, air, everything, then they pop back on and then you will get to feel all comfortable and boom it happens again and sometimes again when you are taxiing.

It seems like everything stops and the sound of sudden silence just destroys any peace in the cabin and people start going Oh no. I HATE that. Is it just a switch of power or something that shouldnt be?

Because they plane uses something called bleed air from the enginges to power all of this. And at the gate itis hooked up to the terminal. On pushback there disconnected, hence the power. The need the power to start the enginges so thats another answer. I'm not sure about taxining though, anyone?

When switching from an external power source to ships power, such as from APU power, the relays that allow engine generators to come on line need to sense a loss of external power before closing. Hence the loss of lighting.

An APU even though it is inside the plane is treated as an external power source by the aircrafts electrical system.

On pushbackbefore starting engines you will also notice your eyeball sockets providing air conditioning aren't working. The bleed air from the APU is being used for engine start, and the AC packs have been turned off.

When switching from an external power source to ships power, such as from APU power, the relays that allow engine generators to come on line need to sense a loss of external power before closing. Hence the loss of lighting.

An APU even though it is inside the plane is treated as an external power source by the aircrafts electrical system.

On push back before starting engines you will also notice your eyeball sockets providing air conditioning aren't working. The bleed air from the APU is being used for engine start, and the AC packs have been turned off.

Perhaps its bad timing when they switch power but my USAirways 757 had the video monitors pop off then on then off then on again during instructions.

Since it was the switch over like you said-perhaps USAirways should wait until disconnected to start their video since the site of a half a dozen monitors going dead doesnt exactly improve ones first impression of a flight!