Before you replace any more parts I suggest that you test the display module and sensor/cable separately.
To test the display module, connect a resistor (any value from 1,000 to 20,000 ohms will do) across the sensor terminals, along with a 12 volt power source across the power terminals (on the rear of the module). If the temperature reading fluctuates or varies considerably over time, you can be confident that the display module is faulty (very rare).
Conversely, if the temperature display remains steady, then connect a meter across the sensor contacts of the connector plug. The resistance reading should be steady (+/- 100 ohms) if the ambient temperature is constant. Also, have your assistant jiggle the wiring harness (while you watch the meter) to determine if you have a loose contact or ground in the sensor cable.
Too bad these tests weren't done before the sensor was unnecesarily replaced the first time. Another thing to check is whether they actually replaced the whole cable or just spliced into the old one. If it was only spliced, the problem may be a bad connection in the original cable that's still there. If they did splice (a no-no) have them redo the job properly.
Hope this helps. Let me know what you find.