"Hi, see my answers below... hope this helps to zooming in on the problem."
Check the cable routing at the fixing bolt on the rear derailleur. The cable needs to run through a small groove. If the cable is on the wrong side of the fixing bolt it will cause strange shifting problems.
"- EDIT: Did you mean front derailleur? fyi - the rear derailleur works like a charm."
I meant the rear derailleur. The problem occurs when the rear derailleur moves the chain to certain cogs on the cassette. I'm thinking maybe the chain is not centering over those particular cogs due to poor rear derailleur alignment. You seem to have tried adjusting the front derailleur several times.
It's not normal to have any chain rub with the chain in the middle part of the cassette.
"- So chain rub should only be happening on the extremities?"
It's more common to have chainrub when the chain is running on a wider angle through the front derailleur.
Al