I bought a used Sigma 70-300DL Macro Super, but it looked like it had never been used. Clean, no buffs or scratches, lens clear etc.

I use it on a Pentax K-5. But it seem almost every shot I take with it, they are oof.

Very frustrating, concerning the reviews and rating for this lens.
I have been using the AF and the live view, the picture looks sharp everytime. Even live view on manual focus, looks sharp. Using the eyepiece or the screen same results. Now I will say that my eyes are not what they used to be, but even with my glasses I should end up with more that what I've been getting. When I up load the pictures to the computer and look at them, so many are just plain out of Focus. I swear that they were in focus at time of shot.
I have tried shooting at smaller Fstops and faster shutter speeds. It doesn't matter, pictures still are out of focus. It is hard to tell if it is focusing short or long. The K-5 has the ability to adjust the focus in camera. I've done that both ways up and down, didn't help. Somewhere on the forum, there is a program that you can boost the adjustments 90 plus both ways, I even tried that and that didn't seem to do much.
I've tried shoot on manual, even then it seems to come out oof. I can't believe that I just bought a lemon. There isn't any marks where someone may of dropped it. I just don't know. I have shot 100's of images through this lens.
My other lenses I don't have any problems with them. Not like this. I use a tripod almost all the time, its sturdy and heavy. Plus I hang something from the center under neath just to help. I'm almost always using the 2 second delay for each shot. I have a UV lens and a C-PL with a hood. I think I'm doing everything right.
So my question is, what do I do? I'm not ready for a float test.. yet.