As far as the text they use for the snippet in the SERPS - its totally random but usually based on the most prominent text when they cache the page from that bot run - or what they match from your title, description and matching text in the page.
Other places they "figure out the importance" of text is from the linking text that links to your pages - if someone links to you that Google considers a good authority site - a lot of times they will use that to match whats on your page that day - thats why its so important to ensure your linking partners use the text you want
As far as your positions ranking - that can be affected by hundreds of things - from how ofter you update the pages, to how often you obtain links that Google spiders, to what other sites get updated for the same keywords etc etc
It doesnt help right now witht them trying to fix a large database update they screwed up on two weeks ago that has kept the SERPS bouncing around everyday - so that part of it is kinda out of your hands