ISP's are good at passing the problem back to the customer. If you can get into your modem settings (try 192.168.1.1 } and see if you get a password prompt. It's usually admin/admin or password/admin or some combination of those by default. If you can, look for something called the SNR margin. This gives you an indication of how much noise is on the line (assuming you are on ADSL/DSL and not cable)
Your upstream SNR should be in the region of 5.0 or less.
Your downstream SNR will be higher but both depend on how far you are from the exchange. If downstream is more than 30 AND you are close to the exchange then it could well be a line fault/line noise. This can be caused by a number of things including heavy traffic, radio interference, power lines and so on.
If SNR changes dramatically over a short period then it's almost certain you have a line problem.
I'm not an expert but I argued this with my last ISP and got released from my contract early without additional charges because of it.
If you are on cable then it's a different story altogether and I don't know much about it.
Edited by webrat, 02 August 2008 - 09:10 AM.