iframe scrolling

[Webskater]

I have an iframe on a page which normally displays a lot of content. I have some javascript on the page which resizes the cell containing the iframe to suit the screen resolution of the user. Normally if you click on the scroll bar below the slider the contents of a window or iframe move up a page. On the page in question the contents of the iframe move up about 2 pages. I am baffled. It happens on some computers but not others running identical browser and operating system configurations and with the same screen resolution.
Any ideas? Any input much appreciated.