combo Box refresh issue

I’m working on a vaadin 7 project in which the comboBox’s data source is set to an SQLContainer, i’m periodically refreshing the container every 5 seconds but after 5 seconds the combox Box goes blank, when i click the drop down agian, it shows the items again and after 5 seconds the list goes gray again, can anybody help me regarding this strange behavior of comboBox ?
Thanks alot