I am having a similar problem to the above posters.
I too have gone through the various upgrade processes since way back when and I never used to get this problem. I must have been using the same modem, router and PCs for 2 years now (Yes my PCs need an upgrade

). I don't know when the problems really began but it must have been around the time the connection went to 10M, lately however the problem has escalated.
Since about the time of the 10M upgrade I have had intermitent lag spikes, lasting maybe 3-5 seconds, this would be irritating but games were still playable it usually wasn't enough to kill the game connection. These spikes have been happening for months now, but lately they've turned into a serious problem causing all machines (between 1 and 3 at any time) to lose connection to the game servers completely. The spikes have also increased in frequency, it's now got to the point where you can only play for 20 minutes then you're gone.
I've been trying to work out where the problem lies and have set up various network configurations.
My original setup was as follows
NTL 120 Modem -> linksys BEFSR31 v2 router -> PCs
This is where the problem first appeared. At that time I aquired a wireless enabled laptop and an XBox so took the opportunity of replacing the router with a wireless version to allow these devices to connect and maybe fix the network problem:-
NTL 120 Modem -> linksys WRT55AG wireless router -> PCs
This made no difference and the problem persisted, the issue then became a real problem with link deaths so I decided to buy a different router brand to make sure there wasn't a common fault with the linksys stuff:
NTL 120 Modem -> Belkin F5D5231uk4 router -> PCs
This did not fix the problem but did something different, with the linksys router I had to force it to renew it's IP (DHCP address) before the connection would come back up, the Belkin recovered by iteself. I then tried
NTL 120 Modem -> USB PC
No problem here.
NTL 120 Modem -> Ethernet PC
Didn't see an problem here also
That's when I called tech support. From what I could understand (quiet line/operator and an indian accent made him very, very hard to understand) the test revealed no problem with the modem, as far as the operator was concerned that was it, not his problem. So I called customer support to get a modem upgrade, got a much more friendly Scottish guy, from reading these forums I was expecting a fight to get an upgrade but he checked my account (Not phoned them in 4-5 years

) and booked an engineer to replace my modem. This was completed this week, so now
NTL 250 Modem -> Belkin F5D5231uk4 router -> PCs
Problem still occurs

I needed a solution badly at this point because I couldn't really do anything, so I replaced the router with a laptop instead.
NTL 250 Modem -> USB Laptop (Running ICS) -> Ethernet 10/100M switch -> PCs
This works fine, no lag spikes and this is the point I'm at now, what the hell is going on? It seems a lot of people are having similar router problems.
In brief (for those who don't want to read all of the above)
The modem has been replaced, the router has been replaced twice, I'm using upto 6 different PCs, but the network consistently periodically losses connection (Not the modem, it's lights always look good). The problem occurs with any number of PC on the router (tried with one/two/three, physically connected), but the modem to USB connection seems fine. What's going wrong between the modem and the router? I've not messed with anything everything is out of the box default.
I'll gladly run any tests, configuration or settings you can suggest that might help resolve this problem.
Thanks
Talifer