do NOT spend that much money, mechanics can be AWFUL people, believe me i know im a girl, they always think they can bullshit me and im like ok peace. they will straight up lie about there bieng something wrong with your car thats not even wrong to make money. do you know anyone that works on cars? you should get a honda civic manual. did you ever get your gauge checked?
ive also got a 95 civic, my temp gauge doesnt work either it just sits at the same spot ever since i got it. even tho i know it has gotten a little too hot before. the way you describe it sounds just like my speedometer sometimes, it jumps all over and jerks around. in fact now that i think about it, my temp gauge was all the way at the bottom once for a few min and then popped back up to where it was again once. it was weird. and if your car isint even too hot when it says it is and its all erratic like that its DEF your gauge. I would keep checking under the hood everytime it goes up high even if you have to pull over.