15-85 canon hard and soft at the same time

Is this a result of the IS? If you look (1 on 1 view), at the letters on this one, you will see the vertical parts of the letters are very soft, yet the straight and sloping downward parts are sharp. Strangely the sloping upwards parts are very soft.!!
Can anyone tell me what is happening here.

I have had problems with this lens but after being told it's down to how i used it left it at that, but I cannot see how I could achieve this effect even had I tried, LOL. Is it simply the IS at fault or have i missed something.

Many thanks