http://www.pingtest.net/ Can test if you are losing packets.
There are plenty of things I can suggest to try and narrow down the problem.
Test directly plugging your ps3 into your modem wired. (If this has no lag its something on your end)
Test going through your router plugged in wired.
Give your ps3 priority in your router. (Set the IP address of the Ps3 to a DMZ host)