I know this is an old thread but the "drop" in clocks is from the 2D clocks like those shown in CPU-Z. Under the graphics tab in CPU-Z, you'll see you can select 2D Desktop, or 3D Applications to view. What you were/are seeing is the 2D settings as 405 Core and 810 Shader are what my GTX550Ti are showing under the 2D tab. Basically, when viewing 2D images (like your desktop or possibly SNES emulated games for example), the card will drop speeds as 900 core (970 or more if OC) isn't needed for 2D stuff.
Also, with my set up I've found that 970/1940 is a rock solid OC without touching anything else and that's 70Mhz over stock. Anything higher than 990/1980 can/will be spotty without touching the voltage. If you decide to touch the voltage, do it in minimal increments ONLY! And even then I wouldn't go more than a couple bumps (best to not touch voltage until you have a full understanding of what you are doing as well as the consequences). GPU's can't take voltage bumps like most CPU's can. I read another post where the kid upped GPU volts to MAX right off the bat and that's a sure-fire way to cook any card. I only use the 970/1940 OC while playing a game, and even then I always make sure the GPU fan is set to max (77 is max with this card using MSI Afterburner).
More often then not you'll see the biggest improvement with games by downloading the most current video driver. Nvidia's most current drivers are usually their "BETA" drivers, which you have to go to their site and manually look for. As of a couple days ago, 3.10.70 is the most current and is giving 26% boost in performance to BO2 for example over the "normal" driver which is by FAR going to give better FPS boost than any GPU OC.