got no _real_ Idea, (and as yet, have not seen a fiat forklift in over 35 years) but with these symptoms, my bet would be a miss adjusted directional switch, in that you don't get zero, or true neutral until the switch is moved towards the rev. position, and so don't have a correct "static return to off" my bet is you will find it is still touching the forward switch when it seems to be in neutral.