Your card is probably running in 2D mode when you are checking it. Run the Furmark test while running GPU-Z and see if it's clocked correctly under load.
The card slows it's self down when it does not need to run at full speed as a power saving feature. It also keeps case temps down.
I just tested it with the Furmark and it didn't go down it was running at 950Mhz with temps at 72C.
But why does it go down when I play some game, that it should never be laggy with this video card?