I think its either a bad program youre using for stacking the images or your camera isnt steady enough. Were there any winds or something like that that might made your tripod move during the shooting?
I was using StarStaX to blend my images. There was no wind, tripod/camera was not touched. I think it's my tripod. Did I mention it's only a $30 one?
Definitely turn IS off...
I'm going to try again tonight without IS. We'll see if that helps. I'll post results.