Interesting, maybe you just got a bad core that won't let you unlock the pipelines. If you think about it this way, unlocked pipelines aren't guaranteed. They are just a bonus that you get. To overclock a video card, only raise the core or memory speed, you decide first. The raise it 10mhz each time. Use 3DMark to test for artifacts. I like using 3DMark01 and using the test called "Nature" to see artifacts on the turtle's shell and what not. The after you find the memory max, do the same for the core speed. Just monitor you temps and you should be good to go.
HINT: Get aftermarket cooling and bump up the voltages to achieve an even higher overclock