Running the a/c, when I increase the rpm's, say for going up a steep grade...I start getting my air flow through the heater (under dash) vents. When I let off of the accelerator, all goes back to normal.

What gives?
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1985 240 DL, 1989 245 DL

Thanks for the replies everyone. I shall go see.
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1985 240 DL, 1989 245 DL

Yes, I have this problem too. I replaced the vacuum check valve and trimmed the small vacuum hose back for a better fit and it still happens. It must be something under the dash as mentioned below, but I've been too lazy to fix it.
It's probably also the cause of my poor idle due to excess air entering the manifold. Got to fix is soon.
I have heard that this can be caused by a cracked vacuum resevoir, also.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon

Dave Shannon's website has good explaination with pics that describe this very issue and how to fix. Further explains Jim's answer. Her is the link http://www.homestead.com/volvo2/Ventwierdness.html.
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Bryan Edwards 1989 240 DL wagon 193k, Silver, Black interior

When you start to climb a hill and step on the accelerator, the vacuum in the intake manufold goes down. It is this vacuum which operates the small actuator which controls the air flap in the heater/AC system. In your case there isn't enough vacuum to keep the flaps in position to direct the cold air through the dash vents.

The problem could be a leaky vacuum connection under the dash or from the engine manifold to the dash. There is a small black and white one-way vacuum valve in the vacuum line from the manifold to the dash. It is usually right up on top of the manifold and easy to see. This could be at fault. My experience has been that the rubber vacuum line gets stiff and cracks. The last time I had this problem, I just replaced this line from the manifold to the vacuum reservoir under the center console and all was well.