Just tried it again following your instructions. Clicked on each combobox and verified the number displayed is the same as one in the dropdown. Scrolled down one at a time selecting each new combobox displayed and verified the number displayed is the same as one in the dropdown. When I reached the bottom did the same thing scrolling up. At every step I did not see the problem.
Maybe you could try updating the SDK to the latest (3.5a at http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+3 or http://opensource.adobe.com/wiki/display/flexsdk/download?build=3.5.0.12683&pkgtype=1 directly)
