Temperature gauge!

[87 national]

I have a vdo temperature gauge that goes up to 250. The other day when i took my car out i noticed it was running a little warm. Normal it runs about 200 in the summer with the a/c pumping, well the other day it was running 225 230 so i shut the car off just figuring it was 110 degree day it was running a little warm. Didnt think of checking the scanmaster, but to continue I took it out today and i just crested a hill and i look down at my gauge and it reads 240 so i drop the car into netrual to shut it off then the gauge drops back down to 210 215 rapidly. Note it did this bouncing around thing like 6 times from 200-240 in the matter of second. So i clicked over to the scanmaster and saw the temperature was 190-196 and the gauge was reading 225. So that explains my running hot a few days ago. The gauge is off, but my question is what would make the gauge read wrong and bounce around like it did today the sending unit?
Thanks

 

[mikestertwo]

Bad connection somewhere...ground...bad sending unit.

 

[novaderrik]

take the sending unit out of the intake manifold and scrape off any thread sealer that is screwing with the ground path. check the wiring- put a good terminal on the wire that plugs into the sending unit and where it hooks up to the back of the gauge.

 

[87 national]

thanks will look into it tomorrow. When i looked down and saw that 240 it scared the s*** out of me.