I have a 2005 Blazer 4.3, today my temp. gauge went pinned all the way to the right well past 260 mark,so I turn the car off hit the top of my dashboard above the gauge and the needle went back to the normal 100 degree mark, I restarted the car and gauge went to working normal for about 10 to 15 seconds and then went back to the 100 degree mark,so this is were I am at now, any suggestions. Thanks in advance !

UPDATE
Earlier in the day I had removed my negative battery cable,so I removed it again and the temp. needle pegged all the way to right well past 260 degree mark like it had been grounded out,so I hook up the neg. cable hit the top of my dashboard the needle went back to 100 degree mark,I then turn the Ign. key to on let the computer reset for approx. 10 seconds, started car and the gauge worked fine.Started car again this morning and gauge is fine. WEIRD !!!
Hope maybe this will help someone else. THANKS !!

 

sounds like either the sender is going bad- easily tested by ohming out. The lower the temp, the higher the resistance should be. The higher the temp, then there will be less resistance. example: 123ohms = about 210°F if the it reads higher at 253ohms = 170°F coolant should be.

Another possibility is a short to ground somewhere to that sender but it may be easier to prove sender problem or not first. Good luck and us updated!