Combobox problem in .Net PDK

Hi,

I am working in .Net PDK 1.0 patch 3. I am facing this weird problem with the combobox. I use a dropdown to list the cities and a combobox for the ports. When the user selects the city, the combobox must be populated with the ports in the city. Additionally, the user can enter a new port too. First I select the city and the combobox is populated with list of ports. I click on the down arrow of the combo to view the ports. When the list is viewed I click on the dropdown. Now a postback happens and the page goes blank. I am not even able to view the page by clicking back. I had to click on the refresh and load the page again. Could anyone tell me what the problem is?