90 ac problems

[nscbarry@mail.msy.bellsouth.net]

hey all, I've found a 1990 90, not a q :(, that i think i'm going to buy
tomorrow. The car is super clean, really tight, with only 54k on the
clock. The one thing that seems to be wrong with it is that the ac when
switched on in auto mode blows from the proper vents, dash, but when the
car begins to drive the air switches to the defrost vents, the floor
vents or the dash and stays at the location it chooses to blow from.
Additionally if the ac is set at LO temp, and low fan speed, the air
flow returns to the dash and remains there. The woman who's selling the
car told me that her mechanic believes that it is some sort of vacume
problem. Has any one had a similar problem with the older auto ac unit
like this?  Could this possibly be the control unit itself? How about
fixes, the unit does still cool very well though even in the 90+
heat here in new orleans.
Also what should the oil pressure be under normal acceleration, seemed
to be a bit high at 4-5 bar?
The idle also seems to be a bit rough, not at all unpleasant in fact i
like the sound of the engine, just not as smooth as my A6. Is this just
a character of the I5?
Obviously i'm going to have to get a Bible, bently, for this car so what
is a good source for one, local barns&noble?
Thanks in advance,
sean 96A6q, 90 90 soon