Horizontal bands with high ISO in low light pictures

I was experimenting with low light photography and the performance of my A1. After couple hours of experiment, I found that if I take pictures at high ISO (ISO800 mostly but few times, ISO400 too), the pictures will have horizontal bands that is a little brighter or darker than the rest of the pictures. You can see the samples from the links below

http://www.photo.net/photodb/presentation.tcl?presentation_id=242533

There are two pictures in the links. They both from a RAW file. You cna see the dark bands just above the dog head. The pictures are taken with ISO800 at a low light (Halogen lamp). I tried taking pictures in a similar situation (but with fluorescent light but don't see the bands).

Does anyone know what happen to my A1? Is this normal? And what causes this effect? Thanks.

Mocca