1988 Bronco Temperature Gauge Problems

Hey, I could use some help as to what to do so I thought i could turn to you guys. I am only 17 and this is my first truck I bought for myself and don't have that great of automotive knowledge and I'm looking for guidance. So the problem is the temperature gauge is pinged on hot even when the engine first turns on so it is obviously not hot. It also still reads hot even after disconnecting the battery and thermostat 'cable?' under the hood. I took the dashboard apart to reach the gauge and was able to push it back to cold and it stayed but then after running the truck in my driveway for a few minutes nothing happened so I tapped on the gauge once and it flicked up to full hot again. Do I need to buy a new thermostat? Is it a faulty gauge? Any advice is welcome. :uhoh:
Thanks

It's a faulty gauge. In an '88 Bronco, count your blessings that's the only major problem. If your engine overheats, you won't need a gauge to tell you that, you'll be able to smell it. Just keep an eye on you coolant level. Pop the cap off the radiator (when is cold of course) and make sure it's got plenty of coolant.