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As Rasta said it probably the guage, or more precisely the wiring connectors to the guage. Clean up the main connectors to the inst panel and all the grounds in the footwell.
Not related to your problem but did you change your thermofan switch when you went to an 80 thermostat. They are matched, a 71 thermo goes with a 75 switch and an 80 thermo takes an 85 switch.
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