Time capsule - internet connection problem via wireless

Question

Hi all - I've browsed around many other similar threads about people having difficulties with their Time Capsules, and although some of the problems I'm experiencing are similar, there's a few interesting oddities that other questions don't address or solve.
I have a 1TB Time Capsule that's acting as a router for my network. It's connected via the WAN port to a Netgear DG632 modem/router (in modem mode) that is set up with a static IP address and the DNS server for my ISP. The TC is also set up with these same static IP/DNS details, and is set to share the one public IP address.
The TC serves up DHCP addresses to the LAN devices via a mixture of ethernet (to a G4 Mac mini and to a Linux-based MythTV machine) and wireless (to a couple of 802.11g Airport expresses, a C2D Macbook using OSX Leopard and Windows XP, an iPhone and a Wii).
Everything on the LAN is happy, gets a unique IP address, can see the other devices, and can see the USB hard drives that are connected to the TC.
The weird behaviour comes when things try to actually connect to the internet.
Anything that connects via ethernet to the TC works perfectly. Connecting wirelessly however doesn't work properly - but not in a simple binary working/not-working manner...
If I connect wirelessly, I can do any of the usual network diagnostic things - e.g. Ping, Traceroute, etc. and data gets through and back, using IP address or actual site names.
I can download mail (via the standard Mail application, v3.5, from Gmail using IMAP)
I can even load secure webpages (i.e. those that use https, such as https://mail.google.com)
What I CANNOT do however is load any "normal" webpages (e.g. http://mail.google.com)
When loading a webpage, there is a brief period of data transfer (e.g. Camino reports "looking up ...", "connecting to ...", "waiting for ...") and then it just stops, and eventually the browser times out.
Other problems that people have reported have been solved by changing the DNS servers used - either to specify their ISP ones directly, or to use something like OpenDNS. My problem doesn't seem to be related to the DNS servers - to my mind, this sounds like some kind of outgoing port-blocking, but there ain't no such beastie on the TC as far as I'm aware, and since it is only happening on any device that connects wirelessly I'm pretty confident that I can rule out any local settings on a particular machine, or any settings on the modem.
I've tried several things to try and fix this, as follows:
Reset TC and modem
Factory-reset TC and set up again
Downgrade TC to 7.3.1 and set up again
Re-upgrade TC to 7.3.2 and set up again
Remove all wireless security (usually use WPA/WPA2 Personal)
Change wireless mode from 802.11n (802.11b/g compatible) to 802.11n only (2.4GHz)
Change wireless mode from 802.11n (802.11b/g compatible) to 802.11n only (5GHz)
Change wireless channel (Automatic, 6, 11)
Obviously none of that worked, so can anyone think of anything else that I can try and change to get this working?
For what it's worth, everything was happy until Thursday last week (11th Dec) and the only thing that I changed was an upgrade to Safari on the Mac mini, which seems extremely unlikely to have been responsible for causing such pain and anguish!
Thanks for bearing with me through all this - hopefully I've covered off all of the basics already, but I'm now not sure where that leaves me...
Cheers!

Answer 1

Something somehow fixed the problem a couple of days after my post - although I didn't know what triggered it (like I don't know what triggered the failure). I changed my set-up so that the Time Capsule was just a wireless bridge, and used the DG632 as the router and DHCP server - and obviously something worked OK.
Unfortunately the same problem happened again the day before yesterday, and after roping in the help of my ISP I determined that there wasn't a problem with the connection outside my LAN, so it was obviously something weird with my set-up.
After much frustration, rebooting, resetting, and fiddling, I spotted that occasionally a small burst of data would be received - perhaps enough to load an icon or small image - before everything froze up. On a hunch, I started playing with the MTU - and glory be! After the MTU is reduced from the default/automatic 1500 to 1458 everything works perfectly... except the Time Capsule no longer shows up!!! ARGH!!
I don't even recall ever seeing an MTU setting on the Time Capsule, and there definitely isn't one on the DG632 (don't most recent devices not require MTU tweaks? They just pass on the whole packet as-is?) so I've got no idea how to get around this issue now.
So much for stuff just working