I have a 2005 Honda Civic LX which has an infrequent overheating issue.

About two months ago I had taken it to my mechanic to flush the coolant system, replace the thermostat, replace the radiator cap, replace the hoses, and complete a pressure test - which kept the pressure so no head gasket problem?

I live in Chicago so now that it is getting cooler I am using the heater a bit more. There are times when heat does not blow on the "hot setting." I asked them to take a look at this about a month ago and the cooling fan works and comes on at 200 degrees but they said something wasn't connected correctly which they fixed so then the heat had been blowing correctly. I have driven the car several times since this last mechanic visit including city and highway traffic.

This past weekend I drove about 330+ miles one way and had no issue at my destination which while in town there I drove about 60-70+ miles city/highway mix because I got new tires and needed to get a lug nut check. No problems. However, driving back to Chicago (about 300+ miles on highway) my last 45-60 mins in heavier traffic my thermostat gauge started climbing. I would turn my fan to high and to the hottest setting which would make the needle drop to normal range. About 2-3 times the hot air would stop and make the needle creep up. I would play around with the cold/hot knob until hot air started blowing again. Once I got into Chicago proper and on the surface streets my gauge stayed up at the "H" and would not drop even with my climate knob fiddling. I pulled over at a park, turned off the car, opened the hood, and let the car rest for 20-30 minutes until I could touch the top radiator hose was not super hot anymore. I looked at the coolant reservoir and it was at the full line so it seemed not to be burning coolant? I turned the car back on and it had a rough turnover. I gently drove the car to my destination. This morning I made an appointment with my mechanic to look over the car. On the way there no overheating issues. The mechanic had my car all day and it did not overheat for him in either the road test he completed nor just letting the car sit idling in the shop for the hour+ to see if he overheated. I asked if maybe it was the cooling fan, but he said it worked properly and turned on at 200 degrees. I asked if it was maybe the water pump but he said that it was fine, and there wasn't any leaks. I asked if he did another compression test and he said he flushed the system again and the pressure stayed. He was at a lost and said it was a mystery. Keeping an eye on it of course and if it overheats taking it to the mechanic ASAP. Any thoughts or suggestions in the meantime would be hugely helpful!