I think you're probably on to something with it being related to temperature. The air ducting to the defroster outlets and some of the dash vents runs pretty close to the back of the cluster so the cluster could be affected by them. Maybe when the HVAC temperature is cranked up and/or the car has been running for a while and the inside of the dash gets good and warm, the heat is letting a solder joint open up and lose contact, or the heat is knocking out that little power supply unit for whatever reason.
Trying another cluster sounds like the simplest next step... if a different cluster has no issues then you know where the problem is. I think those early '84 clusters are somewhat unique, but any year 700 series diesel cluster would probably plug in and work at least to test with.
Good to hear that car is in good hands! Would be fun to see it again one of these days.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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