Hi There
Just a quick thought as to a possible explanation to the softness of your pictures, are you using your lens right at its limit?
No lens is ever at its best right at its extremes, this effect is generally worse at the upper end of the zoom & with len's that sits lower down in the companys product range (its one of the things you get what you pay for!).
Its a simple fix, just use less zoom when taking the picture & crop into the image when editing. With the modern SLR having the quality of sensors they do I've never found a problem doing it!
Its an idea anyway! If it's not the case I'd suggest it would be a good idea to have a look to see if anyone with the same kit was having a problem on the net

(if you've not already done so)
HTH