The cruise control could be the amplifier, control unit, or something as simple as lubrication on the throttle linkage and replacing a burned out or marginal contact bulb. The AC could be functiuoning normally depending on the position of your control buttons - which are a little non intuitive compared to US cars, or it could be a monovalve, vacuum control, or pushbutton control unit. The trans slip could be as simple as a cable adjust and fluid/filter change, or as complex as a trans rebuild. The steering could be poor allignment (4 wheel), bad front suspension bits, tire issues, or most likely of all, rear suspension control arms/bushings.
Each of the above is discussed at length in the archives for detailed info:
Cruise control; including info on how to test and where rebuilt units can be found (try FastLane at the top of the page first). http://www.gdl-online.com/test021.html
Transmission; search for threads with words like slip, Bowden cable adjustment, shifting, etc.
Wandering at speed; search for thrust arm and control arm threads.
AC problems; search for vacuum actuator, monovalve, vent, push button controls.
Probably about a dozen threads on each of these topics - good luck with your projects!