Faulty Heat Gage?
 

My 1989 roadster's heat gauge seems to be erratic! At times it seems ok showing about 175 degrees at cruise, then it will quite suddenly start to jump all over going as high as 299 degrees and the red light come on. Then jump back to 224 and back up to 260, etc. In seconds the gauge will read 194 then 256 and so on. I stopped the car several times when showing readings in excess of 260 degrees and I can put my hand on the radiator cap and radiator and it feels normal. No boiling over or steaming. What is at fault? The sender or the gauge in the dash? Is it possible for the temperture to vary that fast in the running engine? Any Ideas?