@backdraft7, I know you've worked hard to confirm that your rankings are maintained, when your conversions fall off.
If traffic levels are the same, that sort of rules out DNS hijacking. But since traffic just fell off too (I hope you're talking only about Google referred traffic here), then DNS hijacking and cache poisoning does come to mind. See [webmasterworld.com...]
I'm also beginning to wonder if there might be intermittent technical problems on your site. When search traffic stays the same but conversions drop off to nothing, are you sure that your pages are still loading completely? all the buy buttons are showing up and working correctly, etc?