Try disconnecting everything, then plug in only your modem (via a micro filter) and see how it performs, if its still bad try another microfilter, if its still bad then its either your line, computer disconnecting due to timeouts or simply phone your ISP.
If however it does work, plug in your other devices, phones etc, one at a time (making sure to use micro filters on each) and see at what point it breaks.
It may also just be a loose connection between one of your cables / micro filters / extensions.