First you have to try to forget that it worked well for time it does not work Now so something is wrong.
Step 1. Computer by itself connected to the Broadband modem.
If No Good check Network settings and Optimize the connection.
If still No Good call the ISP.
Step 2. When Single computer Good while directly connected to the Modem, plug back the Router and try again.
If No good check all Router?s settings especially the Router?s Keep Always Alive and MTU settings.
If Bo Good you might consider getting a New Router.
Step 3. If Good plug the Xbox, if bad with the X-Box, and the configuration is OK, borrow an X-Box from friend to rule out problematic X-Box.
If Good then problem solved.
:sun: