You've got a vacuum problem, and it's pretty common for commodores around this age. You lose vacuum under load (accelerating up hills, etc), and with a vacuum leak you don't have enough vacuum to keep the face vent open and it defaults to the floor.
Forgive my memory, but the Berlina has climate control, right? The symptoms are the same whether standard or climate, but with climate control the fix is slightly different. It's more than likely to be a problem with the solenoid valve pack (hidden behind the console just below the radio). Check out the following thread for guidance:
http://forums.justcommodores.com.au/showthread.php?t=22750
Just remember the leak may be in the solenoid pack or it could be in any one of the vacuum lines coming from the engine. Your choice what you tackle first:
A) Replace the solenoid pack @ $130 new (take you an hour or so) with no gaurantee the leak has been fixed, in which case head to the next step.
B) Check all other vacuum lines for leaks (it will take a couple of hours but it costs you nothing unless you have to replace a hose). If you don't find a leak anywhere then go to A).
Hope this helps.
Mark.
