Not seeing this problem in your capx. Here's a screenshot running in Chrome, zoomed in. It's blurry because of the zoom but there is no noise compared to your initial example.png
<img src="http://i.imgur.com/XCSJS.png" border="0" />
ETA: Testing was done on a PC with just the built in intel graphics chipset. Preview mode only, not exported. I checked the PNG inside the capx with Photoshop and it's perfect, no alpha channel problems there.