Belkin F5D9630-4 G+ MIMO Modem/Router: LAN connection from PC drops after about 1 minute

I purchased a new modem router to increase coverage.  I have exchanged the router twice - this is now the third one all showing the same problem.  I can start the router and connect and configure it, and get an internet connection, and view some web pages.  But after about 1 minute the wired LAN connection from PC to Router drops.

If I unplug and plug in again, I can connect again - but only for 1 minute.  I can see the IP address is correct via ipconfig, and can ping successfully when I restart the router, but after a minute I can't ping it again - even though the ipconfig still shows OK.

I have tried with obtaining IP address automatically, and with setting my own IP address but without success.  My PC is windows XP, service pack 2.

I have added a new Belkin ethernet card, but that has made no difference either.  I have tried more than one cable (including the one that was working on my old router and is working on it again now - see below)

I also have a windows 2000 laptop, which I have added the Belkin G+ Mimo USB adapter.  I am unable to make a connection as it is unable to see the wireless network, although it does show up as an available network (not sure how better to explain that bit)

I have spoken to Belkin over many days.  I have tried with the firewall off and on without making any difference.  The call with them has now been escalated to a level 3 technician - I am hoping to get a call tonight, but have been having problems with this service so I am seeking my own solution here.

Last night I reconnected my old router to regain internet access, and unplugged the dsl line from Belkin router and plugged it into the old router.  Having done so I then pinged the Belkin router successfully.  I was able to ping it for about 30 or 40 minutes, when I went to bed.  There were a couple of timeouts when I was inserting and removing the wireless USB adapter in the laptop, but they disappeared after a few seconds.  The laptop could also see the wireless network at this stage as well.

When I plugged the dsl line back into the Belkin router, I was unable to ping it, but could again when I unplugged the dsl line.

When I left for work this morning I tried pinging again, but timedout twice, however I phoned home and got my wife to try, and she was successful.

Sorry about the length of the question, but I thought too much information is better than not enough.  I hope someone can help.