If the fan regulator transistor unit fails, it will most probably always show the same behavior,
either always blowing ful speed, or not blowing at all. If the fan sometimes blows and
sometimes not at all, I think the problem may be something else than the regulator.
The regulator (power unit in Vadis) can be replaced seperately, contrary to the engine fan
where fan and regulator are one assembly.
The best thing you can do to prevent the regulator unit from failing is to replace the cabin
filter in time. When the filter gets clogged, the fan needs to work harder, the regulator gets
hotter and its cooling becomes compromized.
Should you try to replace the power unit yourself, I can post a few pdf's describing the job
and a parts list of the fan and regulator unit.
144 GL (1974)--->244 GL (1982)--->940 GLE 2.3i (1992)--->XC70 2.5T (2004)