sigma 400mm 5.6

hi i have a sigma 400mm f5.6 the problem is when the camera is set to f5.6 i get a perfect exsposure but when i alter the f stop to say f8 the shutter speed drops to match the aperture but the picture is over exsposed and it looks like the lens is still stuck at f5.6 with the same depth of field the aperture ring is set to the A setting.I have tried moving the aperture dial when the lens is of the camera and the apertures move freely.The camera is a k10d.Any explanation why this is happening.

I assume the camera works properly with other lenses?

Try setting a small aperture and hitting DoF preview (with preview method set to Optical if you don't usually have that). If the lens is stopping down then the view through the viewfinder should darken.

If the lens is not stopping down, take it off the camera and examine the stop down actuator (this is the longest movable prong which sticks out the back of the lens mount). Compare it to a lens which works properly. A weakness of the K mount is that this actuator is not particularly strong and if it becomes bent then it wont engage properly with the camera and the lens wont stop down properly.