internet quandary

I have a perfectly fine running system on dsl, however, at times i can be on the web without any troubles at all, then go away and come back a short time later and not be able to navigate anywhere. I do know that I am still online cause other programs are still active and my comp still sees the ISP. The only way to get it back is to restart the machine. My question is why does it do this and is there a possible fix?