Just done it on mine.
You want the next option: Interval composite (the end option) and set the interval, no of shots, and start interval. I have the Composite mode on average but you may want one of the others. Use an appropriate drive mode (start interval). That'll do it!
As I said, page 61 of the manual!

Which of the four options have you used to set the drive mode?

Did you set the number of shots and shooting speed first?

First one with the old manual lens is really good. Which lens was it in fact?

Well I know my old K100D wasn't compatible with SDM, but then it wasn't a DS. I'm surprised a major update like that didn't warrant a compete new version number rather than just an added S. I tried a Google search for K100DS and nothing showed up, just Pentax K100D Super. And as we know, all Pentax's are super!

Bottom line then, it seems to confirm the lens is duff.

You can't check the auto focussing of an 18-135 on a K100D as that camera isn't equipped to handle Pentax's DC (or "Direct Current") autofocus motor lenses, and that lens lacks screw drive focussing. They're not really compatible though manual focussing should be possible. The lens ought to be OK with the K-5 but a test on another compatible body might help prove the problem is with the lens and not the camera, given the camera may have other issues.
Is the flash problem just when using the 18-135? If so the camera and lens might well be fine as PM25 reported.
It would probably help if you could visit a specialist Pentax dealer or Pentax owning friend who could check each item against a known working one to identify where any problem lies.

Yes (K-1 page 61)

Thank you all.

You were allowed to keep both if you wanted!
PS Wasn't me.

I have the 50mm version of that and it's a really fine sharp lens.