VW Touran Climate Control Gremlin - Lack of "Umph and air" - Any advice please.

Dear all,

I have a 05 Plate 2.0 TDi VW Touran Sport (7 seat with DSG Box).  From
start up all is well but I have noticed that after a short while the
Climate control / Ventilation system appears to reduce considerably
the amount of air coming out of all the vents - ALL BY ITSELF !, even
when NOT set in "AUTO" mode.

When this happens, and upon turning the fan speed to maximum to try
and get some air flowing out of the vents, there appears to be a
slight increase in vent noise but no increase in flow out of the
vents.  It almost feels as it there is something restricting the air
flow (the air recirculation switch in NOT on, but makes no
difference.) I supect the fan is behind the drivers airbag / bulkhead
area as this appears to be where the noise is, and its not visible
from inside the engine bay.

I have found that turning the whole system off, give it a second, and
then back on again solves the problem as hey presto the air starts to
flow as normal.  It then however takes 5 or 10 mins (sometimes a few
mins) to revert back to the very poor flow rate (that poor that you
sometimes have to open a window !

Any suggestions as what could be the probable cause ???????

Thanks in advance,
Andy.