Seen as you are having the same proboem with the usb modem and the router and in most cases there isnt a problem with your phone wires as long as your not a cut em up and mess around with them type of person, Then i suggest there could be a problem on your computer..
You should try scanning your computer for spyware / adware /malware and even virus infections as these regulary can cause comunication problems with the internet.
Next thing is anti virus software like Norton or Mcaffee, When these are not correctley configured which i have seen in hundreds on complaints on my own customers, this causes internet problems with regards to connecting to websites and using messenger services. It may be worth trying uninstalling these softwares and try your computer direct to the internet without protection to see if this cures the problem, Antivirus software and firewalls can be re added later.
Another thing that might be a giveaway to the problem is things that load up when you turn your computer on. Do you get popups or warnings saying your computer is infected with various malwares? this is also a big giveaway to adware infections.
big programs to remove spyware include Spybot and AdAware , both are free, do a google search for them.
All failing that, call Tiscali and request a detailed line check and also ask Bt for one too, as there may be a problem between the pole and your house, i have sen this too after house works on the gutters has been performed, and bt replaced the outside line resulting in a perfect connection.
Cheers
Andy
June 2008