The 17-85 is not a good lens at all. Get rid of it!!!!
Had exactly the same error on mine no chance its worth fixing. Upgraded to the 17-55mm and it is night and day the 17-55mm is so much sharper, less distortion and vignetting. You also get a constant 2.8 aperture throughout the zoom range which is very useful.
The 15-85mm is much better than the 17-85mm so I would go for that if you cant stretch to the 17-55mm. Gives you more range but is a slower lens but the IQ and distortion are much better.
I would not recommend going for an L lens like the 24-105 or the 24-70, not because of the quality, you cant beat it. But at 24mm on a crop body it must be multiplied by 1.6, so the widest you can shoot on those lenses is 38.4mm. The other option is to buy a 17-40mm but that is still 27.5mm so the only way to get wide on a crop is to stick with the 15-85, 17-55mm. Or buy using two lenses the 10-22 and the 17-40 combo.
Hope this helps