A/C blowing out of defroster on HOT days????

My 1990 350SDL is blowing the A/C out of the defroster on day when it's over 80-85 outside. If it's a little cooler, the A/C come out of the dash vents as it should. If the hot day cools down, the climate control will switch from defrost to dash. Is this a vacuum pod problem or a climate controle unit problem? Any guidance would be great. Thanks!