A/C problem with my 1984 300DT

On rare occasions when I turn the A/C on and select high fan speed the blower is very fast (read powerful) and the temperature is very cold......Great! On other occasions in fact most of the time when the A/C temp is selected and the high speed fan switch is selected the fan is slow and the temperature is warm to cool. Yet at other times if I switch off the engine before switching off the A/C for example when refueling and then turn the engine back on the fan blows H-O-T. I've been told that the problem is cold solder joints in the A/C module and that I should remove the A/C module and resolder or at least reflow the joints. Does that sound right or could it be something else?

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