Electrical troubleshooting can be very confusing and difficult. I usually do the kind of stuff you have been doing for awhile to see if I can score the easy fix. However, at some point you need to start over and pursue it in a very methodical manner.
Here is what I would do. This will involve pulling the dash loose and out some where you can test things.
Steps:
1. Turn on ignition and measure the voltage supply at the temp gauge. This is the voltage provided by the IVR. This should be about 5 volts. If this voltage is wrong or unstable, nothing else matters, it won’t work right.
2. Find the wire connection that goes to the temp sensor. If you short this wire to ground momentarily, the temp gauge should move towards full scale. If it doesn’t, the problem is in the dash.
3. Take the temp sensor and connect it to the temp sensor wire on the gauge with a piece of wire like step 2. Ground the body of the temp sensor to a ground on the dash. With ignition on the gauge should read cold. Boil a glass of water in the microwave and stick the temp sensor in it. The gauge should go up to about a normal reading for a warm engine.
4. If step 3 works, do it again with the sensor grounded to the car frame this time. This will determine if the dash ground is good to the frame.
Tired of typing for tonight. If this is useful to you, I will keep going.
