I am actually running into the same problem

I have a 5k+ gs now and have fixed my gem situation and am running into the 3.4-4k dps range. I have been watching the charts of other locks with basically the same build and class of gear (sometimes lower) and they are hitting 5k-6k dps, I mean that's a huge difference in my book, and no matter what I throw from what I have read so far, I can't seem to break 4-4.5k under the best of situations. I am using a 0/14/57 build and it has done well for me for about 4 months now but will be switching to the 0/13/58 build soon I think.
My rotation is CoD > Immolate > Conflag > CB > Spam Incinerate > watch timers and it varies from there (not including my trinks and Glyph of Life Tap), although some locks now suggest throwing in Corr when I have read and been told by a number of locks that the wasted time casting and keeping corr up is damaging to the other timers considering the little gain if any in dps.
Here is a link to my char if anyone want to evaluate what I have, I realize I have two improper enchants but was just getting something on instead of nothing. Also missing a few enchants and I have one piece of pvp gear but I am working on this :P
Just in addition to minglewood post, I usually keep my FPS >15 in my case...not that that answers anything but just thought I would throw that in there.
http://be.imba.hu
Thanks for your time...