Browser stops browsing

What would cause a browser to quit browsing? Every once in a while, IE or FF will just stop working. It doesn't freeze up and everything seems normal, but it has stopped downloading. It thinks it's still downloading or refreshing though, but if I check Status by clicking the connection icon, Bytes Sent and Bytes Received are not incrementing anymore. Sometimes Bytes Sent is incrementing but Bytes Received is not. There is no pattern to this; it can happen anytime, even in the middle of a big file download. It doesn't happen all that often, thank God, but it is very annoying when it does. I have to log off and reconnect to make the browser start working again. I get no error messages. Since it happens with both browsers, I figure it's an OS or ISP problem. I'm running a Compaq Presario with Vista Home Basic, IE 7 and FF3.0 and I have MSN Dial-up. This never happened with any of my previous computers with Win98 or XP and same ISP. Thanks.