S L O W Internet connection

We have been having problems for the last week with an extremely slow internet connection.
We have a fast ethernet switch (brand new, replacing a 5 year old hub), connecting four computers to a belkin wireless router (few months old) which connects to a wirespeed modem. While we have a low speed dsl package, it's always worked fine and nothing has changed recently to cause this. Everything was working fine until about a week ago. We have tried resetting the modem & router, and that sometimes speeds things up, but only for a little while. Verizon spent an hour doing some tests, and determined that the problem is internal. So we replaced the hub with the switch, which helped for a little while, now it's slow again. It involves all computers whether connected via cable or wireless.

Any ideas??? Thanks.