heater & ac fan cuts on & off

When u start 1984 300SD in morning the def fan or heater will start if u switch it to auto high or low it will also run but then for unknown reason it will stop, sometimes it will start again and if you turn it off it will
not come on again it comes and goes does it have anything to do with the temp. gauge outside and if so how do I fix it , thanks bj
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