When you checked the resistance of the ground(s) - where exactly are you measuring? Did you make the measurements with everything still connected? If you simply measure between the ground of one of the instruments (say, coolant temp) and the vehicle structure, then you are not really measuring just the resistance of that wire, because there are many other parallel paths that exist when everything is connected.
If you think there is a problem with the gauge grounds then you should remove each ground wire (one at a time) and then measure the resistance from the end of the ground wire (where it normally connects to the instruments) to structure. The gauge grounds are likely small gauge wires - on our Audi they are 0.5 square mm (20 AWG) which should have a resistance of about 10 ohms per 1,000 feet. Since your ground wires are likely less than 10 feet long, you should measure something like 100 milli-ohms (0.100 ohms) maximum. If you see higher resistance, then maybe you have a damaged wire or the wire's connection to structure is corroded/damaged.
Your coolant temp sender (or the wire connecting it to the gauge) could also be bad. Your sender is likely a thermistor, so the resistance of the sender goes down as temp goes up. If the sender or the signal wire is shorted to structure, then the gauge sees zero ohms and will be pegged high. Try removing the signal wire from the sender, that will look like infinite resistance and the gauge should drop to its lowest value.
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'85 Audi Coupe G(in &) T(onic)