Like Tandee said, IS(image stabilisation) isn't a panacea to all ills .. it's just a patchup to some specific issues.

You can't compare the use of a tripod and the use of IS.

Reason for asking about tripod(and not IS) is that a tripod will eliminate any fore/aft movement you may naturally be inclined to produce.
A tripod is set into the one spot and that's it.
IS will only assist to reduce fine camera shake issues, not positional movements.

fore/aft movement: if you have the AF mechanism set to a single shot af mode(sorry but I dunno what Pentax call theirs), which is usually called AF-S mode, then what happens is, the camera focuses(step 1) and then you shoot(step 2).
In the moments between step 1 and step 2 can you positively and conclusively say that you haven't moved a millimeter or two in the fore/aft plane(as well as in the lateral plane .. but this almost certainly won't affect focus)

Focus works along a plane, and that is in the fore/aft plane. If you focus .. move a mil or two, and then shoot, then where you did focus was incorrect as you have moved a mil or two relative to the plane of focus.
This will look like the lens is soft, or misfocused, or whatever.

That above falls under the technique heading:
If you are 101% certain that you haven't moved in the fore/aft plane in the moment between focus and exposure, then you are a tripod!

As Am mentioned, your issue could well be an alignment issue between camera/lens. AF micro adjust can help with this to a certain degree. Prety much useless on a zoom lens.

Do the image test shots that Am mentioned.
Be 101% sure that the camera doesn't move .. handholding and relying on IS is no guarantee for that.
If you have no tripod, then just place camera on a rock stable surface of some type. (table/chair/rock, whatever! .. just make sure it doesn't move)
if images turn out a little blurry, try IS, faster shutter speeds or even much lower(as in 10+ sec) shutter speeds, or mirror-lockup or whatever.
Keep ISO to a minimum tho.

And I'll mention it again(as everyone else has) .. post some sample shots with EXIF intact!