Odd problem when downloading torrents

[#1]

I have a PC that just does torrents. It has an IP address from my public block and is bridged through a firewall on an OpenBSD box. The same box does routing and NAT/PAT and firewalling for the rest of my LAN. My modem/router is just in bridge mode - no active stuff going on in it at all.
It all works perfectly, at least I thought it did. I've always set the torrents on a schedule overnight, but the other day I had it running while I was actually there.
Here's the problem: I can barely browse the web from my other PC's when the torrents are running. Pages often just don't load; IE just times out. Mostly they very slowly, partly load; missing graphics or layouts etc.
Note well: there were just three torrents active, about 70KBps down and 15KBps up, less than 40 connections. That's on a full 8Mb connection so it's not hogging the bandwidth...
I put the torrent PC on a switch straight from my modem/bridge - so no firewall in the way. Really I was making sure the BSD box wasn't being overloaded and slowing browsing. No difference whatsoever. I rebooted everything several times.
I can still do binary usenet downloads at full speed (minus what little the torrents are taking) on another PC so there's definitely plenty of bandwidth left. I can download files at full speed too. It just seems to be web browsing that's affected.
I can't figure it.
I've even run wireshark on the incoming line (mirroring the switch port connected to the modem) and one of my LAN PC's. I can see the packets being sent to the modem as they should be, but there's definitely a delay in getting anything back.
Of course, I can't see what's going on on the other side of my modem.
Do the clever folks here have any ideas???
Rick.

[#2]

Not very clear is this an XP box? If it is do you get a Event ID 4226  in the log? Basic Xp with sp2 only lets you have 10 connections so if you havent patched it that would account for what you are seeing. If it is the case you need to get the patch from http://www.janwright.co.uk/tcip.zip.
If your not running Xp I would guess there is a similar restriction on the number of open connection on the O/S you are using it was a big thing to stop the spread of the worm from pc so it could only infect 10 at a time instead of 100's

[#3]

Hi, sorry for the confusion, they are all XP boxes (apart from my BSD router/firewall).
All have been patched to 75 max connections, 1478 MTU and 368,128 RWIN. Routers and modems set to matching MTU as well.
The Torrent PC isn't used for anything else, I'm browsing from another PC. I've tried from other PC's also to check the problem isn't just on that PC.
the problem feels very similar (except worse) to what I get if I try browsing while another PC is doing 20 binary usenet connections.
I have another modem (Bintec) that I'm going to try as soon as I remember how to get it to just bridge instead of being clever. I did it once but forgot how. Last time I tried to remember, I messed about so much that BT dropped my profile and I still didn't get it. It's not documented!
Another thing: the problem persists for quite some time after I've pulled the network wire out of the torrent PC; I think it does gradually improve but it takes a while.

[#4]

I assume you port forward your torrents to the bit torrent PC?
It will take quite a while before torrents stop requesting date from your torrent PC after you've unplugged it.
Is your OpenBSD PC actually doing anything important that couldn't be done by the router?
Can XP actually support 40+ connections at a reasonable speed? Or does it support many fewer whilst keeping the additional ones open (but "inactive" in some way)?

[#5]

No, the ports aren't forwarded. The torrent PC has a public IP and the BSD box acts as a bridge between the incoming line from the modem and a switch for the public IP devices. It's also a stateful firewall.
My modem/router can't do both bridging of my public IP block and NAT/PAT routing for my LAN at the same time, hence the BSD box which can. It's also fun to play with.
It's not the torrent PC itself that has the problem - I've seen it do 10 torrents at once with over 500KB/s down.
Edit:
Hmmm - I've just got out of my pit and remembered what to do with the Bintec. It's starting to look like the old Solwise has developed an 'issue'. With the Bintec in it's place as the bridge, I'm downloading a couple of torrents right now and can browse the web at full speed as well.
And there was I not but a few hours ago singing the praises of the Solwise in the 'What router do you have' thread. I must have jinxed it 
It's strange though. I used to use the solwise when I had a single IP as modem and router and it never had any trouble keeping up with torrents, usenet and browsing all together. Oh well...
(Having said all that, watch it all go pear shaped as soon as I post this