Just a guess, but try to back your core down to 230 and test it. When you up the clocks too high, 3dmark score drops because it is becoming unstable. Although it may be stable in games, 3dmark will reveal the fainter signs of instability. Also, the memory clock is much more important than the core clock for performance.
You have to find the sweet spot, where you get the highest 3dmark score (then join the overclockers 3dmark team!), and decide if you want to run it maxed out, or if you should give your hardware a break and a small safety margin by backing off the settings a tad for actual use.