Temp Gage moving
 

I have an 04 explore with a temp gage that has a mind of its own [:@]. It will spike up to the H and then just as quick drop down to normal.
I changed the thermostat but it didn't help at all.
Also I have just noticed that there is no heat! I was trying to defrost the window this morning and got nothing but cold air?? I shut it off and waited a few minutes and then turned it back on and hot air? Then it went cold again. Anyone with any ideas please?