If you watch YouTube (or other flash) videos with hardware acceleration turned on it will drop your mhash in half.
If you go to Youtube, right click on any video and choose settings. Then on the Display tab (far left) uncheck enable hardware acceleration.
That will keep your rate from dropping when you watch the videos.
You can also up your rate by overclocking and setting flags as stated here:https://en.bitcoin.it/wiki/Mining_hardware_comparison
Just find your card in the list and you'll see what everyone else used to get their speeds.