Well, since nobody seems to want to take a stab at it. I remember reading a long time ago that if you only had one speed for the blower fan the blower resistor on top of the HVAC suitcase (under the dash) was the blame. Perhaps yours is being affected by heat and is on it's way out.
x2 My 68 /w A/C fan speed dd not work. I bought a new blower resistor ... problem solved