Having a nice system doesn't always equal running without lag, especially when you are talking about a java based game. Java can do that from time to time. There are a lot of reasons why, but like snow said read that article, and get optifine once it gets updated...
I occasionally get lag but more often don't, seems to be more to do with my internets than anything else.
(Internet in aus can get a bit patchy)
Internet often "a bit patchy" in Australia. Especially during the holiday season when all the kids download the latest movie, music or games. The down/uploading to YouTube. Then there the video chat for hours on end, in HD! Around here, the internet can sometimes go down to a crawl for the first few days of the holiday season. Internet on optic fiber, yeah that'll happen.
I seem to be lagging a little more than I normally would but I've put that down to not having Optifine atm.
WoM Coins: 3691
Give your JVM more access to your RAM and give it a higher CPU priority.
I've already explained your issue in this thread.