I bought a 04 Honda civic with 180K miles on it and the vehicle has some overheating issues. There doesn't seem to be any leaking and I got the radiator pressure tested. The previous owner replaced the thermostat recently and I notice the temperature overheating becoming more of an issue as it is becoming hotter. The temperature will stay at the normal temp for about 45 minutes of driving and when I experience traffic or I wait at stop lights for a while, it will start to rise to about 3/4th temperature and then bob back down to normal temperature. It will continue to do with throughout the drive until I allow it to cool off. The temperature gage never reaches the max high temperature and typically experiences this when it is hot outside. I also have noticed that my coolant is corrosive.
Have you checked your nearly two-decade-old radiator for clogs? If your coolant is corrosive it needs to be changed. Don't assume the new thermostat is good, assuming it was even really changed.
Also, see the FAQ section on diagnosing overheating.
would you recommend a coolant flush first?
With a radiator that old if it's clogged I would just replace it.