First of all did what heliotrope suggested help you at all? I hope so, but if not please tell us some more about the problem.
*Does this happen at any paticular time ie when your checking your mail or not using the internet?
*Does it happen at paticular times of the day or randomly?
*How do you connect to the internet ie do you use a shortcut or do you connect straight through a program like internet explorer or outlook express?
*Is your modem an external or internal motherboard? (Can you see it?)
*Have you tried anything to fix it ie rebooting the computer?
*Is there any audible noise or static on your telephone line when you are talking?
*What is the exact name of your modem? (Find this in your Device Manager)
*Did you install any software reason or can you think of anything else you might have done before you noticed this problem started?
If you can answer as many of these questions as possible then you can help us help you better but in the mean time reading this webpage may offer some helpful things to try...
It is worth going through anything that evenly remotely applies to you because this document its full of excellent and helpful advice.
And if all that fails if your ISP (Internet Company) has any kind of a heart they will offer you a free helpdesk. Contact the helpdesk and I'm sure they will be happy to help you with your problem.
Hope that helps for now,