Can’t always surf

I am running Windows XP Home Edition. My ISP had some problems over Christmas and one of the symptoms was you could log on but no surfing and no email server could be found. They fixed the problem but I continue to experience this phenomenon on an irregular basis. I can log on but no surfing and no email server. The problem is solved by rebooting. Works every time. It usually occurs after several logons and logoffs and if the computer has been on awhile. One of the advantages to XP is that you don’t have to reboot much so this is even more annoying. What’s my problem?

Thanks

Mark