Enormous recurring spikes in number of visitors

[sbook]

For the past month our website has an enormous increase of traffic. But this increase stays only 30 minutes. Google drives 5 times more visitors to the same pages, which usually drives us traffic. We can see this on the graphs online. Usual number of online visitors is 700. On the spike it can be 1800-1900. I have cleared cookies, used proxy to check google position and they are the same as previously. We get this bounce around each 3 days. Actually when we see daily UV graph, this spike is not visible. Only in daily online graph. Any idea what should i do, what changes should i make, to "save" this traffic? Any experience with that? Same situation was with our other websites, but after a month or two it just stopped and we stayed with same traffic. It is not country related.

Graph attached
[i46.tinypic.com...]

[Robert Charlton]

sbook - Thanks for the observations. What have you noticed about whether the traffic converts?

Please compare notes also with this ongoing discussion...

Zombie Traffic and Traffic Shaping/Throttling - Analysis
http://www.webmasterworld.com/google/4437835.htm [webmasterworld.com]

[sbook]

Yesterday we got again some spikes.

[i46.tinypic.com...]

Earning stats show these visitors were real.
Same pages had visitors like usual day. Just number of visitors was double.
Google Serps in all countries were the same.

I still cannot understand where visitors came from. Looks like G usually show us in Serps only to 30% of people. And on spike they show us to all.