1. disconnect the yellow wire under the intake manifold to the sensor and hold it to the engine block with the key on in the 2 position

answer: if the gauge goes all the way to the top red line you have nothing wrong with the gauge itself. if it only goes up halfway or as you describe rep-lace the gauge

2. if on step 1 the gauge goes up all the way under this test your comp board is failing and either needs replacing or better yet. bypass it by soldering a jumper wire between pins 1 and 3 after tossing the board

3. if the problem remains after bypasing the comp board replace the sensor mounted in the cylinder head.