If your shutter speed is down around 1/20th. (and you regularly find yourself shooting with a shutter speed slower than 1/90th.) then I'd be looking at camera shake or mirror slap as the culprit. Are you aware of the hand-held shutter speed guidelines for lens focal lengths? Basically, you should keep your shutter speed around 1/focal length. 1/30th at 28mm, 1/90th. at 85mm, etc. when shooting from your hand. Image stabilization lenses can help here, but there's also mirror slap - the vibrations initiated by the mirror when it hits the top of the mirror box. Some cameras are more susceptible to it than others. It manifests itself at slower shutter speeds. You might want to try setting your ISO such that you keep your shutter speeds higher and see what that does for your image sharpness.
Another thing you can try is to measure off an exact distance, set the lens to that distance, shoot, and see what comes out. That is, use a tape measure to measure exactly 8 feet (and place your subject on that spot), set the focus mark on the lens to 8 feet, put the camera on a tripod, set the shutter to 1/60th. or faster, shoot, and see what you get. If that's not sharp then I'd have my lens looked at. Next, leave the camera on the tripod, set the camera to autofocus, and take another shot. If the image is blurry at this point then the autofocus in the camera probably needs adjustment. If, on the other hand, the image is still sharp then I'd begin to consider operator error.
My image was taken from ~3' away with a 120mm macro lens on a medium format body, equivalent to a 75mm lens on 35mm. Magnification was around 1:5 life size, so yeah - close in with a telephoto.
People who do stupid things with dangerous substances often die! -me