Hi Giorgio,
I don't think the engine timing wiill effect the gearbox slipping. However, the timing, points and plugs must be in good order and properly setup. Then the fuel injection, throttle linkage, etc must be setup correctly. Before fiddling with the gearbox linkages.
Read up and do the 'linkage tour', which will include setting up the CSS(constant speed solenoid)
Now read up as much as you can on the gearbox linkages, the throttle switch, the 3 way solenoid and the kick down switch.
Good luck.
Chris