aros71:ubergeeknz: Do you have other things happening at the same time as you try to do this - eg torrents?
PLEASE don't disable the Firewall.
Not running any Torrents, P2P or similar.
Internet-connected devices on the network: 4 PCs, iPad, iPod touch, couple of smartphones, a Bluray player and a WDTV box, plus a skype phone. I don't think the number of connections is the problem...
Regarding the firewall. My thought was to drop the firewall, set up a DMZ host, and attach a 2nd router to this, then attach all the network devices to that. Lighten the load on the Genius as much as possible. I have a Dlink wireless router but it can't to VLAN tagging on the WAN side else I'd try putting that straight on the ONT.
Again, DO NOT disable the firewall. Doing so opens up the Genius DNS server to be used in DNS amplification attacks. This is Very Bad.
We'll need to do some testing on this but I'd be surprised if it's a widespread issue as it's the first we've heard of it. I'll let you know soon.