i had a similar problem lately... turned out to be my phone line! still... i rang orange and went through a load of tests with them and it ended up that they were going to get BT to do a line test. i rang back the next day and the test came up 'clear'... so i gave BT a shout myself who automatically told me that i had an earth fault on my line
complete waste of time calling them imo, they can only offer basic advice, most of which i had already tried to no avail.
NEILM - is the left hand light on the modem flashing or solid? i had a problem with my username when i first got broadband... didn't realise it had to have '@fs' at the end of it!