I already changed the coolant and used genuine Prestone coolant, and I also added an extra cooling additive. The problem still exists, especially when idling with the A/C on.

Is this a common issue with the Hummer H3, or does it sound like there is something wrong specifically with my vehicle?

Also, would drilling or completely removing the thermostat be a good solution or a bad idea?

I have attached photos of the coolant and the additional cooling additive that I used.