Draytek 3220 Send empty respond

Hi,

i have DrayTek Vigor 3220 Series. and i have two ADSL internet connection aggregated with session based. but the problem  that the connection randomly dropped and if I try to open a web page it give me (empty respond). and if I did a test speed it give me only the speed of one wan connection.so can any one help me to figure it.

Thanks in advance.