If it actually stays pegged, I would be sure to have proper anti freeze. Your problem can indicate a thermostat stuck open. Easy to test. Pan of water on the stove with a thermometer. See what temp it opens and what temp it closes. Opening temp should near match the value stamped on T stat. You could also have one that is rated too low. You could have a bad new gauge. Tube kink somewhere. Pegging at left is worth doing these tests. It is good that it goes up to 150-180. How does your heater work? Good? Did gauge readings match thermometer when tested?
When you take out T stat, observe it, it should be closed. ALL THE WAY closed. Observe for crusty. Observe for upside down install.
If all checks out, run with radiator half covered with cardboard to see if things change.
All above is for peace of mind but may discover something.
