My 1990 760 turbo had whay sounds like the same symptoms. Going up hills or accelerating, the cool air would quit coming out of the dash vents. Turned out to be an internal leak in one of the vacuum servos specifically the dual servo onthe left side of the console . One fix is to cap off the side of the servo that's leaking; or you can replace it. There's a section in the 700/900 FAQs on this that shows how to replce the servo without removing the blower unit. I thought maybe there was an electrical cut-out, but it was the servo that fixed it.