Test by unplugging your temp sensor and shorting it to ground and watch the gauge, it should jump to hot and stay there and not move. You'll find it just behind your ignition cap and before your #1 injector, there you'll find two sensors that look the same. The one closest to the injector is for your computer and has two terminals the one next to it has only one, this is the sensor for your gauge.
Also depending on your year, check the main engine cannon plug for corrosion.
If these tests fail, your SI board could be causing the problem.