Why does my car temperature rise when I idle but when i drive it, it's goes back to normal?

So I started noticing this problem 2 days ago. I thought I fixed it by refilling the coolant. It was working fine till today again. When I stop on a cross street or I idle, the temperature slowly rises up to the "H" but once I accelerate or move the car, it slowly goes back to normal. My A.c works... show more So I started noticing this problem 2 days ago. I thought I fixed it by refilling the coolant. It was working fine till today again. When I stop on a cross street or I idle, the temperature slowly rises up to the "H" but once I accelerate or move the car, it slowly goes back to normal. My A.c works perfectly either way. What could be the problem? How much would it cost? It's a 2005 Honda civic ex. Thanks.
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