Curious MF problem on 60D with 18-250mm Sigma lense

Hi, 
It seems Sigmas are known for quite a few focusing issues but I've scoured the internet and not found this one.

When switching my 18-250mm Sigma DC HSM to manual focus the camera sometimes has trouble recognising it has been switched. Consequently when trying to take a photo it will only take one in perfect focus once the focus confirmation light is solid green. Effectively rendering manual focus pointless - very frustrating!
It doesn't happen all the time either, as I typed this I left my 60D on next to me and when I just picked it up again it had recognised it was in manual focus ("M-focus" on top display) and I had no problem taking pictures. 
A quick switch back to AF and then back to MF and its still stuck in AF (to be clear that means I can select One-Shot, AI Servo etc on the camera but the lense just makes some unhappy mechanical noises and doesn't change focus)

The lense is out of warranty and if there's some fix for this problem anyone's help would be greatly appreciated!