Temperature gauge in my car not working at all

It stays on cold entire time and doesn’t move. When I take the negative terminal off off of the battery, the temperature gauge will work for maybe up to 30 minutes and then fall off and stay there. I changed the ect sensor thinking this was the problem, but same exact thing so it’s not the ect sensor. Car is not overheating or anything but it does seem to be sluggish and getting poor fuel economy. I don’t understand why resetting the computer would allow the temperature gauge to work for 30 minutes and shut off again. Thanks for the help in advance.