Car overheating when driving up hills / idling

Hello, I have a 1995 honda civic with 210,000 miles. It has been running great for a long time. recently I moved from Missouri to California and on the way it started overheating when I would drive up the mountains of Nevada. I got some of that additive that cools your radiator 20 some degrees and was fine for a couple months. Now it is starting to do it again, and I’m using this additive once a week. Its also starting to overheat when driving in the city for more than a half hour. It seems to stay cool as long as I have that additive in it UNTIL i drive up a mountain or large hill. which are hard to avoid in San Francisco.

Usually the radiator is shot when that happens. Check the radiator from the rear and if you see white or green coloring all over the bottom of it, you can have it changed just because of that. If you can see the fins between the tubes and some of them are rotted away, change the radiator. If you can pull the shroud back, you will be likely to see a lot of them missing.