Why is dual wan router software losing connectivity?

I'm using Edimax dual wan router software to load balance of two broadband connections. I have WAN1 connected to a Zen Internet modem router, and WAN2 connected to Sky Broadband. From time to time PC’s on the LAN will lose connectivity to web sites. They can be working fine one minute, then suddenly cannot connect to web URL’s. When the problem happens they can sometimes ping the DNS server, and can access mail servers, but web pages are not found. If I disconnect either of the two WAN connections the problem goes away, and the PC can access web sites again. If I wait for a few minutes the problem also usually goes away. Does anyone have any idea what might be causing the above problems? Please help.