Lens compatability with ist D

On my ist D, I use a Tamron 28-105 f4 AF lens with no problems. However, I have an older Sigma 28-200mm f3.5 (I know, not the best quality, but buying the camera left me short!) that doesn't function properely on it. It works fine on my PZ-1, but on the ist d, it never focuses correctly, exposure is fine, but not focus. Has anyone else dealt with this? Anyone find a solution? HELP!

Funnily enough, I have an old Sigma lens which focusses perfectly on my Z1-P and*ist D, but not at all on two different SFX bodies.