I have a strange heater problem on my 1993 Chevy Caprice Classic SW. If I select "heat" on the heater control. I have hot air blowing out at my feet, just like is should. Everything works just fine, until there is a load on the engine. For example, ff I'm going up a hill, the hot air then comes out of the vents in the dash. As soon as the load off the engine, the hot air then comes out of the heater vents again. At first, I thought it was the heater control malfunctioning, as I have had problems with those in the past, but it is definately associated with load on the engine. Most likely a vacuum leak somewhere. Any thoughts? Ideas where to begin to find the leak? Thanks