I've had my 99 Bon ssei for about 5 years now. Every summer, when driving on interstates, in hot weather, the temperature gauge slowly creeps up over 200. I never let it get 230. I'm able to get it to go back down by turning off the AC but sometimes I have to reduce speed. This is most noticeable at interstate speed 70-80, but it was really bad when driving in the mountains at low speed/high rpm. The condensor fins are in pretty rough shape. I straightened them once, but after one winter they looked about the same. As far as I can tell, everything is working fine under all other conditions. I was contemplating replacing the condensor and or the radiator, ,maybe a high flow t-stat,or have others noticed this? My 93 never did this. LIM gaskets, Tstat, hoses, water pump (mostly NAPA components) have all been replaced, and I have no reason to suspect the gauge is bad.

Sounds like a clogged something somewhere. Try replacing your fluid with a chemical flush. if that doesn't work see if your fans are working. does your heat work?. You might have a cracked somewhere on the radiator or a bad cap this would keep pressure from building compromising the effectiveness of the cooling system. Try that check list on for size.

Second on the chemical flush. I ran a bottle through with my last radiator flush, and even though I wasn't having anything close to overheating problems, it still brought the temperatures down. Even in late spring, I couldn't get the temperature back up to 195° for two weeks. Driving highways on a cool night now cools it down to 170° or 180°, and that'* with the stock thermostat.

It'* cheap -- I'd recommend it for everyone. Wish I could remember what brand I used, but I got it at AutoZone.

I'd look for airflow restriction to the radiator/condenser. Is the deflector there on the bottom of the core support? Any junk in between the radiator/condenser?
I once found a Walmart bag stuck between the rad/cond causing an overheat problem with the A/C on.

I like Prestone anything for radiator. Also check to see if your cap is leaking. I mean it should be doing it now but try sanding the radiator filler neck a bit where it meets the cap.

Some kind of reduced flow, either coolant or air. Check both out, and flush the cooling system.