What's wrong with my 24-70?

Hi,

I've had an EF 24-70 lens for a while now, but only just now started noticing how bad it is at 24mm. I'm talking high shutter speed, stopped down - not focal curvature problems at f/2.8. I know 24-70 isn't the best zoom on the block in terms of sharpness but what I'm seeing is simply unacceptable quality. Here's a 100% sample (caution: 11 MB) - it's at 24mm, 1/250 and f/8. Does this seem normal to you? If not, is a sharpness, decentering or some other issue?

The thing is throughout the range it otherwise seems quite acceptable - especially at the 70mm range. Wonder whether to send it for a checkup and calibration... Thanks.