My guess is that there is a specific program that could be eating up your bandwidth and without actual data it is harder to debug.

There are a lot of different ways you can see what applications and services are eating bandwidth.

• nethogs is a per application based solution


• ntop is an overall statistics solution


• netstat is also a good way to see which services are connected


• bmon is a nice real time monitor

I would also check your router logs and see if Ubuntu is trying to use some services from your router, but being denied or generating errors.

Once you post more information I can try and debug further.