A big misunderstanding of Google and search engines like Yahoo & MSN is to view them as “one big super-computer.” In fact, they are tens of thousands of machines, located in different “data-centers” (DCs) all over the world.
And they do not get updated all at the same! Instead, changes are rolled out slowly, a few data-centers at a time, and a few machines per datacenter at a time.
As a result of DNS-based load-sharing, the “Google” you connect to right now is not the same “Google” you connected to five minutes ago — It is a different machine at a different IP address so different set of results.
So, you are simply seeing results on different Google machines, depending on when you connect (and where you connect from).
If you see your brand-new site appearing and disappearing, but ranking well with increasing frequency, that is potentially good news.
On the other hand, if you see your well-ranked site dropped with increasing frequency, then that is bad news. It is however possible that you’re connecting to only partially-updated servers (computers), and your data isn’t loaded yet. It doesn’t make sense to panic until your site disappears completely, because it might drop, or it might pop back — You just can’t tell.
This is why no SEO company in Scotland can promise you No.1 in Google. From minute to minute, even Google engineers don’t know who will be top for a specific search term on a specific computer / datacentre.
We aim to build good quality sites with quality incoming links to ensure at least your site remains bobbing about on Page 1 of the results.