it can be so frustrating being able to surf the web fine yet not able to play lfs.
couple of thoughts, is the cable connecting the router to the filter ok unravelled completely? ( I imagine if your anything like me you've removed it and re-inserted a few times), and the patch cable to your PC is that ok? any kinks? or loose plugs ( you might, have a cable tester built into the network drivers on your PC)
Is the router in a hot place or maybe somewhere with electrical interference? (try turning off any low voltage and flourescent lights or anything that could be suspect, I have read that the router should be at least 2m from any sky or sky+box) try moving it if you can to the extent of the cables reach, worth a try.
Have you changed any settings in the router setup page?, have you upgraded the routers firmware? you could try a full factory settings reset and then re input all your login etc.
I take it your router has a built in spi firewall, they normally do, if you want to risk it for a very short time, just to test, try disabling it but only for short test, you understand, at your own risk.
I wouldn't rule out phoning the tech support (watch out for those 0871 numbers often £1 / min +) and explaining the situation they should be able to see the errors/discos on the logs, and can alter your s-n ratio and conn speed from their SW you may find a lower speed will give much better ping stability, as the more data pumped down the line = more noise too (stupid BT copper shite) anyway worth a shot as last resort, however the people trusted to change your settings are normally not the after hours/weekend crew.
might be you have a fault on the line (does look that way I'm 3k+ away from exchange, stable at 3-4kbps conn speed) again tech support should be able to do a proper line test or try BT on 151 (free I think)and ask them to do one.
could even be your new ISPs traffic shaping kicking in, I have read that an LFS packet looks very similar to a P2P packet, not sure about coD or bc2 tho.
Well thats a few thoughts sorry its somewhat of a jumble of Qs
good luck.
SD.