I been getting erratic air temp on my 92 400E. All other gages work except the water temp. When the car is cold, it will raise to normal temp without any issues. Once there, the temp will occassionally jump around, even higher than 120C. Then sometimes it will drop back down. I assume there is air in the system, but how do you get it out? Is there a bleed valve?

__________________
93 Toyota Corolla 177K
95 S 500 coupe 82K
92 400E 180K (SOLD)
94 E 420 150, blown gaskets (SOLD)
http://www.kanio.net/mercedes_signature.jpg