Not to minimize your problem, it's probably just a temporary glitch that should disappear on its own. How long have you been seeing this?
As you've probably noted, depending on the number of results per page, Google will cluster results from the same domain differently. To describe this for the benefit of other readers... if you have two results on a page with the default setting of 10 results per page, Google will move the second result up and indent it below the first.
A site that ranks, say, #1 and #10, will (with the default 10 setting) get positions #1 and #2.
If you're normally at #11, as you increase the default results-per-page setting, more of the sites ahead of you are likely to be showing multiple results and your position will shift down.
I can't think of a worst setting to disappear on, though, than the default 10, but, as I said, I do believe it is a temporary glitch. Google is pushing around a lot of data, and display problems do happen. I'd give it four or five days before I'd really worry. It's up to you whether you want to contact them now or wait.
For problems like this, I usually wait.
[edited by: Robert_Charlton at 8:25 pm (utc) on Nov. 10, 2007]