erratic temp gauge reading... pls help...

i have a 94 s10 blazer with the 4.3L v6 cpi... the problem is my temp gauge . within 3 or 4 mins of driving the blazer the temp gauge starts to raise and it will raise all the way up and peg out.. it reads 260.. and as we all know u can tell if you vehicle is running that hot.. it runs great and never over heats.. after i drive the truck on a 1 or 2 hr trip or something like around town it will drop and read correctly but i am so afraid it is going to overheat one day and i will not know about it and it will hurt it worse.. if you have any ideas pls let me know.. all i know is the temp sensor has been changed but when i took the radiator cap off it had a gel like substance on the bottom of the cap .. my first thought was a flush but that did not help.. pls help me figure this one out..thanks

I would flush the system out and change the thermostat. You said that you replaced the temp sensor. You have 2 temp sensor. One for the Temp Gage and the other for the ECM. Another way you can check your temp reading is by using a scanner in real time and select engine temp. Check your scanner temp against your gage readings.

thanks for the suggestions.. i am not sure which temp sensor they changed it was on the side of the block ... also how would i go about flushing the system.. i know how to change the thermostat but have never flushed a system .... i don't have a scanner but if last resort i will take it to a shop and have that checked ... if you have anyother suggestions then pls let me know thanks

The sending unit for the gage is one wire. I've had problems with after market sending units giving the wrong reading.
Use an ohm meter and check what ohm reading you are getting.