Loss of internet connection

[#1]

At exactly mid-day today, after working successfully for some time, I was unable to access any websites, although Outlook Express was still working OK. The MSN Messenger troubleshooter said there was no Default gateway.
This has happened on a number of occasions over the past six months, although sometimes Messenger reports that the DNS Server is unavailable. It has happened at various times of day, although at least four occasions have been exactly at mid-day. The only solution appears to be to restart the router, which incidentally is always switched off overnight.
I have a D-Link DSL-G604T router, which appears to show everything working correctly. I have the DNS configuration as "Use Auto Discovered DNS Server Only" with no Preferred Servers specified. In the past I have tried declaring Plusnet DNS servers or OpenDNS, but it seems to make no difference.
Can anyone suggest what might be causing this please?

[#2]

Does anyone have any suggestions please?

[#3]

When the problem occurs I suggest you run ipconfig  /all in a command window. (Start->Run then type cmd in the box and click OK.)
What you get should be something like the following. This shows whether your PC's connection information, including its Default Gateway (which is usually the router), is valid.
If all entries are valid try typing ping 192.168.x.1 <router IP> and ping www.bbc.co.uk. Do these work?

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.
C:\>ipconfig /all
Windows IP Configuration
        Host Name . . . . . . . . . . . . : George
        Primary Dns Suffix  . . . . . . . :
        Node Type . . . . . . . . . . . . : Unknown
        IP Routing Enabled. . . . . . . . : No
        WINS Proxy Enabled. . . . . . . . : No
        DNS Suffix Search List. . . . . . : Mabel
Ethernet adapter Local Area Connection:
        Connection-specific DNS Suffix  . : Mabel
        Description . . . . . . . . . . . : VIA Rhine II Fast Ethernet Adapter
        Physical Address. . . . . . . . . : xx-xx-xx-xx-xx-xx
        Dhcp Enabled. . . . . . . . . . . : Yes
        Autoconfiguration Enabled . . . . : Yes
        IP Address. . . . . . . . . . . . : 192.168.2.2
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 192.168.2.1
        DHCP Server . . . . . . . . . . . : 192.168.2.1
        DNS Servers . . . . . . . . . . . : 192.168.2.1
        Lease Obtained. . . . . . . . . . : 15 July 2008 14:31:07
        Lease Expires . . . . . . . . . . : 15 July 2009 14:31:07
C:\>

[#4]

Thanks. I'll try that next time it happens. Incidentally when it is in that state, I can still log into the router and it appears to show everything is OK.