tedster - 5:36 pm on Dec 9, 2007 (gmt 0) Pure theorizing here, by the way.
Might this be a traffic related filter? I'm thinking that there are traffic criteria for staying on page one of the SERPs (click-through rate, fast back button, etc) and for some reason on the specific search involved, this url doesn't meet those traffic metrics when it hits page 1 - even though by other algorithm metrics it "earns" a page 1 spot. So it gets caught in a loop of sorts.
Pure theorizing here, by the way.