Sigma HSM compatability

Has anyone heard or had experience of the compatability of earlier Sigma HSM lenses with the K5-ll/lls? I bought a non OS 120-400mm which works perfectly on my K-30 but will sometimes refuse to lock focus on my K5-lls. It's rather as if you've put a tcon onto a slow lens - the af just goes from one end to the other without locking. The screen also seems to look a bit dimmer through the viewfinder. I've done all the usual cleaning of terminals etc. It's odd that it will sometimes seem to work and sometimes not. All other lenses work perfectly on the K5-lls including a very old Sigma non HSM.

I've not had any problems with the latest 150-500mm on a
K-5 even with a Jessops (clone/type) 1.7x TC.