Engine Temperature Guges goes high when going uphill

Hoda Civic 93 140,000 miles

Hi,

I relpaced the new radiators, hoses, and themostats on my civic about a year ago for coolant leakage and it has been gonig fine.

But recently, when my car going uphills, the engine temperature guage would go up to around 3/4 and go back to normal gruaduall ( which is about a quarter high) when it goes back to normal ground.

I could not find any leakage on coolant, and when the radiator cap is open, the coolant was not boiling, and the fan is running.

What the problem might be? (The weather is a bit hot recently like 105 F).

Thank you very much