what could cause this?

[ame="http://www.youtube.com/watch?v=7kpCAVEcLLc"]YouTube - amp/volt meters[/ame]
As you can see the amp meter swings between pos and neg, and the volt meter also shows increase/decrease voltage. The problem is sporadic and varies, sometimes bad enough to cause the lights to dim when the amp meter swings neg, and voltage drops. what can cause this to happen?
Thanks!