Q: Temperature gauge goes up to 3/4 then back down to half.

This kind of behavior may mean that you are low on coolant. Normally the tip of the temp sensor is submerged in coolant. When the coolant is low, there may be an air pocket around the sensor that allows it to get as hot as the surrounding metal until the coolant starts circulating and covers the sensor. Check the coolant level at the radiator with the engine cold and idling and the heater on. Top it up with 50% coolant and water mixture. It may never have been correctly filled after the radiator replacement or there may be a small leak in the system. To have it fully evaluated, you can contact Your Mechanic to have a technician come out to your home or office to check out your temperature problem and let you know what is the solution.