GMC Repair: temperature gauge, blown head gasket, engine coolant

Question
My truck('95 GMC Sierra with a 305 cu. in., auto trans.)overheated recently. The temp gauge was all  the way over on red for about 5-10 minutes before I could get to water.  I was traveling on a highway and tried to get somewhere for water.  I managed to get  water in it to get it back in normal range.  I did not shut the engine down, it ran the whole time.  I don't have any leaks and it is maintaining a full level.  My problem is that the temp gauge needle is going erratic.  The needle on the gauge is constantly moving from one end to the other from the time I start the cold engine until I turn it off.  I changed the thermostat, thinking maybe it was the problem but it still acts the same. I am not driving the truck more than a mile at a time because I am afraid to burn up the engine.  What might I be able to do to correct this myself. Thank you.  

Answer
My next proceure would be to remove the radiator cap after the engine has been fully warmed up and look for any signs of air bubbles in the radiator and or any continous expelling of engine coolant all the time when the engine is at an idle speed. If this happens, This is a sign of a BLOWN head gasket.

BE EXTREMLY careful when removing the radiator cap on a fully warmed up engine as there may be a TREMENDOUS amount of PRESSURE when removing the radiator cap and you can get SEVERLY BURNED if you get splashed with hot engine coolant.

Let me know and be CAREFUL when removing the cap.



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