Trouble creating stars with Noise + Levels at hi res

This is something that I've never come across... I tried all sorts of methods of fixing the problem, but, to no avail.

First off, I'm designing a 24" x 36", 150 dpi poster. There is a part in the poster where I want to create the illusion of space and stars. Easy enough, just make a new layer, fill it black, add a Levels adjustment layer, tweak the black and white sliders, and voila, stars....

I took a screenshot at 12.5% zoom: http://i234.photobucket.com/albums/ee180/ClaytonPerryman/Screenshot6-1.png
This looks exactly how I want my poster to look.

BUT, when zoomed in any further, more noise starts to become visible for some reason...

here are some examples:

16.7%: http://i234.photobucket.com/albums/ee180/ClaytonPerryman/Screenshot9-1.png

33%: http://i234.photobucket.com/albums/ee180/ClaytonPerryman/Screenshot10-1.png

66.7%: http://i234.photobucket.com/albums/ee180/ClaytonPerryman/Screenshot11-2.png

and finally, 100%: http://i234.photobucket.com/albums/ee180/ClaytonPerryman/Screenshot12-1.png

These are all with the same levels, mind you. All's I'm doing is zooming in a bit each time.

As you can see... none of these look much like the 12.5% rendition I linked to at the beginning. It's mind boggling to me. Am I missing something?

Basically, my technique for creating the stars is making a new layer, filling it black, add noise (Amount: 110%, Distribution: Gaussian, Monochromatic: Checked), new adjustment layer of Levels, tweak the black and white sliders... and that's it.

Computer specs, just in case:
MacBook Pro (Intel)
Adobe CS4 Master Collection (Photoshop CS4 specifically in this case.)

Sorry for the long post, it's sort of a hard problem to describe without being in front of someone. Thanks in advance for any help or advice you might have.

It's the scale you're working at. A technique that works great at 4" X 6" isn't necessarily going to work out at 24" X 36". This technique is one of them. What you're going to have to do is build your star background at a smaller, more workable scale, flatten it, and then slide it into your larger image. My guess is that you'll have to experiment a bit to find a good balance for correct black/white contrast and star size - I'm also going to guess you may have to add a bit of gaussian blur to your star background once it's in your larger image in order to take off some of the pixelation you'll no doubt get by enlarging this image to size.

Good luck!