I think we need a bit more info also.

Is it always the same client that lags? or is it random clients each time you play? Does the client that lags have any game lag such as video chunkyness, etc? Are you running XP or Vista? If XP, have you done the "latency trick" to drop your ping times?

I had a similar problem to this that kind of popped up out of nowhere. My general latency on 4 clients would be high but still ok (~150ms or so) and 1 client would always be 500-900 - was never the same client, seemed to be random.

My cable modem had gone bad - as soon as the cable company swapped it out for a new modem my latency on all 5 clients is <70 always, even during peak times. This is with the latency trick (search this forum for latency trick for details).