sporadic overheating

I'm having a weird problem that my local dealer service can't figures out. Car runs fine until you drive at freeway speeds for 10-15 mins and then exit and drive stop and go. Once you get off the freeway the engine overheats. We replaced the thermostat, didn't fix issue.

If you drive slowly (no freeway) then it runs fine. When you're on the freeway, it runs fine. Only when you exit and slow down in traffic does it do this.

Any ideas are appreciated. I'm going to start replacing water pumps next...