Why does my 2007 impreza 2.5i overheat when driving up mountains?

A year ago (roughly 10k miles ago),
My car always overheats when climbing up hills at freeway speeds (65-70mph). Usually with 2 other passengers and lightweight amount of cargo.
The dealership performed tests on the entire cooling system and found no external leaks but found combustion in coolant and blown head gaskets. They ended up replacing head gaskets, thermostat, engine seal, and reassembled heads.

Yesterday is the first time I drive up the same mountainous region going to tahoe since the repair (about a year ago) and behold my engine temp begins overheating. I had 2 passengers this time as well and it happened immediately after the incline began. I turn around and drive back home not wanting to risk damaging anymore internal parts, with everything back to normal now.

I do not have any other problems or symptoms with engine overheating except when I drive up the mountain and the engine thermostat seems to rise 3/4 the way up and no further at which I immediately pull over and cool engine down. Coolant starts pouring out of reservoir.

Can anyone tell me why it is doing this? I have a little over 81,000 miles on the car.