Have you scanned your pc for viruses or any other form of malicious code? try using, AVG Free or Avast, spybot s&d, spyware blaster and a-squared free. You could also use ccleaner which a good little app to clean up your system.
Have you checked
your area for any issues?
Also, is your modem compatible with the broadband package you have chosen? I know my old Motorola SB4100 was not therefore, i had to upgrade.