1993 Chevy Caprice Classic SW, Strange heater problem

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

You have it right. It's a vacuum leak somewhere. You will have to run the lines under hood and behind the dash. I'd look for a failed vacuum resevoir under the dash somewhere....

Run the engine + heater. Listen for a vac leak with a piece of vac hose to your ear, and the other end wherever you suspect a leak.

That's a good idea, a homemade earscope. I need to find the time to do this soon. Summer is almost here, but I don't want to let it go until December! Thanks, I'll post what I find.