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
Engine Test Stand Playlist: https://www.youtube.com/playlist?list=PLwoxyUwptUcdqEb-o2ArqyiUaHW0G_C88 restoring my 1968 Firebird 400 HO convertible (Firedawg) 1965 Pontiac Catalina Safari Wagon 389 TriPower (Catwagon) 1999 JD AWS LX Lawn tractor 17hp (my daily driver) 2006 Sequoia 2017 Murano (wife's car) 202? Electric car 203? 68 Firebird /w electric engine 2007 Bayliner 175 runabout /w 3.0L Mercuiser__________________________________________________________
