What causes the temp gauge on my chevy venture to respond I Radically?

My temp gauge registers normal when the car is first driven. Once the engine is turned
off and restarted, the gauge reads very high. To lower the gauge reading, the starter has
to be tapped without starting the engine.  This sometimes does the trick but not always.  
The indicator at times passes the red zone of the gauge yet the engine is not overheated.  
What causes this?