Sure you can fix it. You will need to know how it works and how to take it apart. From a wiring diagram you will see that the power and ground circuits for those two gauges are the same. You might assume one of these to be the problem (more likely than the individual parts). A gauge has three wires (circuits): power, ground, and a variable resistance sensor signal.

Before I disassembled anything I would try to verify my hypothesis. Since the power and ground are internal, the sensor signal would be an alternative. I don't off hand know the resistance of the engine coolant sensor, but I would read the thing and then substitute a known good resister of similar resistance and a good ground. Drive the car a see if the condition exists. If the condition continues the dash must be removed.

All the circuits can be powered individually but this is time consuming. The 126 dash uses the overlaying of components to continue the ground from side to side. Loosen and retighten all the mounting screws that pass through multiple components. Identify the nuts mounting the individual gauges to the printed circuit and loosen, retighten them also.

It is very common to have fuel gauge needles swing madly due to the sensor in the tank. It is uncommon for the temp gauge sensor to have this problem. Chances are good that a circuit common to both is weak. Either find it or replace the gauge section (after verification by substitution at sensor).