I have a 2003 LW300 lately when I start the engine, the temperature gauge pegs all the way to the right and the over-heating light comes on. It eventually returns to normal but occasionally it will peg all the way to the right again and the warning light comes back on. It can last from 1 second up to 10 to 15 seconds. I know that changing the thermostat is a serious pain, so I wanted to know is there a sensor that I can replace first to see if that is bad before trying to tackle changing the thermostat? Any help would be appreciated.

 

I'd try checking the engine coolant temp ensor for starters.
Posible loose or shorting wires may be creating the erroneous readings. I don't think they are real; your coolant temp could not instantaneously peg out High for 15 sec then come right back to where it was