Fluctuating Google UK Rankings Every Alternate Day

[mrperfect4all]

One of my sites was ranking well in the SERPS for past 6 months(UK Search). In July suddenly the ranks disappeared,and then they bounced back, but every weekend the ranks used to disappear in SERPS. Now every alternate day I am loosing the ranks and then regaining position. Loosing the rank here refers to the site completely disapearing in the SERPS and then suddenly the next day all ranks come back.What do you think might be the problem?

[tedster]

We've got another thread discussing this - we're calling it the "Yo-Yo Effect", although it has been called the roller coaster, the sine wave, and other things too. There's no defintive understanding of the issue so far, but some ideas are developing in that discussion.

In order not to have two thread on the same topic, I'm locking this one and we'll take all the discussion into the other thread.