Would anyone know why, the aircon on my EL switches from venting the cool air onto the drivers face to venting the cool air onto the windscreen when the motor is under load. ie. driving up a hill?

I think it could be a vacum issue but I have know idea where to start. I brought the car brand new in 1998 and it didn't do it when it was new.

Any help would be great.
Thanks Billy

It is a hard one to find ,mine has been doing it for ages.Ckeck your vacuum lines coming off your intake,behind your glovebox and worst case your aircon vent switches.Good luck.Kevman