bandwith problem

Hi,
i have a problem with 2 ASG in HA, i have a 4M WAN fully used but i don't know .



I tried iftop via ssh but i can't see the source ... I have no access in reporting tab because it's too slow .
If I reboot the ASG 220 the problem reappear few hours after .

Somebody have an idea to check the problem ?

Best regards,
Guillaume