Car overheating randomly even after thermostat was replaced?

I have a 2005 Honda Civic lx. It was overheating and brought it to my mechanic. He said the thermostat was sticking and replaced it. He also checked the head gasket and fluid and both were good. After about 4 days, my cars temp garage climbed all the way to the top. I turned the heat on to lower the temp and it blew only freezing cold air. It finally started to blow hot air 10 minutes after I had it on and the temperature gauge fell back to normal. My mechanic ran tests to see if I had a leek but didn't find anything. They had it for 2 weeks and couldn't get it to overheat no matter what they tried. Now here we're a week and a half later and it did it again. I was driving home on the highway and the temp climbed all the way up and had to pull over. Shut my car off and restarted it after a few and blasted the AC. The temp fell back to normal for about 30 min. In the last 10 min stretch it began to heat up again. I pulled over, waited for a bit and it came back down. It stayed down until I pulled into my house. I took it out for a short 2 mile drive and it stayed normal. When I got home and parked and tried the heat, it was freezing air. Not sure what to do since this keeps happening and is not consistent. Any suggestions? (Keeping in mind I have like zero car mechanic experience/knowledge)

You have the right idea but turn the heat on not the AC. Ac will add heat to the rad as it is in front of it. At this point I would take out the t-stat. Couple possibilities here. Bad flow thru rad. I would say rad fans but highway speeds are enough flow thru the rad to keep it cool. Water pump fins worn. Coolant level low. Head gasket leaking into the cooling system.

The problem is only cold air comes out when I use heat. They checked the head gasket and said it was fine. I think they did a test to see if I had any leaks and it came back negative

Check hose

If the radiator fins all bent? If so maybe a clogged/bad radiator it happens, also check the fans

Key word: 'Randomly' overheating. This means it comes and goes, not following set patterns or repeating under the same conditons - so that rules out possible causes like a degraded radiator, a leaking head gasket or a slipping water pump. Those causes do not come and go 'Randomly'.

The electric cooling fan, its control circuit, the fan relay, and the coolant temperature switch in that circuit -that is where cooling problems occur Randomly.

And another odd possibility with random characteristics: The engine coolant temperature sensor and its signal circuit could also have a connection problem.