it wont be alternator or cas or anything random like that
it will be whatever is tapped onto the RPM wire
so piggybacks, other systems etc
in my case with the exact symptoms you had, was the RSM was being random
it could be the RPM wire (like in yours) is either damaged, mangled, a botched RPM hack previously etc
we both have a powerfc, active lsd, series 2 GTST and the same problem, RPM need goes spazzo between 0 and real RPM