Do you have any other film negs? Try a neg from another film type and see if the cast remains. I doubt if the film is to blame but you need to eliminate possible causes. I may have misunderstood your cast categorisation but you have mentioned different casts. When you say an instant change from too green to too magenta are you saying you change the M filtration by say as little as 1-2 units and the change to the opposite cast the occurs?
Filtration with Fuji paper is in my experience more critical thanwith Kodak paper but while I have seen a change with as little as 2 units ( Durst Dichroic head in my case) it is quite subtle and not an instant reverse