Okay I still have no clue what it could possibly be ?! Here is a list of what happens

1) Temp gauge will go up the O of normal then right back down to cold then go up a little then go all the way to the L and stay there for a minute and goes back down.

It does not over heat when it says its hott.
I have a new Thermostat
My heat cuts out sometimes!
And when i put it in neatral and drop is back down in drive the temp will go down when its all the way up.

Other then that the car drives fine?!

 

is it low on antifreeze?

 

do those have a sending unit? I've had the sending unit go out on an oil pressure gauge I had in a past car that made it go whacky

 

you may want to flush the sustem. What color was the coolant when you looked at it? Rusty?