Problems with dual wan router

I'm using an Edimax dual wan router (BR-6624) to load balance 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 (it doesn’t matter which one) 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.

I've noticed that this problem only seems to occur with PCs which have a static LAN IP address - I haven't yet seen the problem on a machine with a DHCP-assigned address.

I've also found another issue when I have both WANs connected - and this one affects ALL PCs on the LAN. I am unable to connect to certain secure sites (e.g. banking sites). When I move from the home page to the login page I get a 404 error. Again, if I unplug either of the two WAN connections the problem goes away.

Does anyone have any idea what might be causing the above problems? I've referred them to Edimax tech support but haven't had a reply yet.