Overheating

The car started overheating on a trip to Utah the outside temp. Were 102. I was driving quite slowly at the time - about 20 mpg- and the temp. Gauge was almost in the red zone. I turned the a/c off (which was blowing warm air by now) and drove a mile to our hotel. Called a dealer and they looked at it and replaced the thermostat and flushed out the coolant and replaced it. They took it for a test drive and said it was good. So I took it and had to drive through some "stop and go" traffic and the temp. Gauge started rising again but as soon as I got on the freeway, it lowered and was fine. It did this again the next day when I got in slow traffic but on the freeway it was great. We drove the 600 miles home with no problem and have had none for the past 3 weeks, but we live in a rural setting and don't do a lot of driving. Today we drove to the city - about 150 miles and it was a hot day. It started overheating in the same manner as before - only when we were in slow traffic going 35 mph or less. I stopped for a few minutes and kept the engine running with my husband in the car and came back to steam come out of the hood and coolant streaming out under the car! I turned the car off (temp. Guage beyond the red zone!) And lifted the hood - the coolant reservoir was boiling excessively and kept at it for at least 10- 15 min. When it had cooled down some we decided to see if it would run - ran great at 45 mph or above but the temp. Guage would climb up to the red zone anytime we had to slow down. Drove home at speeds of 70 mph with no problem - replacing the thermostat and flushing the coolant system did not good - is this more likely to be a fan problem? Because I am a woman, the men in the shop seem to not think I know what I am talking about - I know if they take it for a test drive around here, it is not going to show the problem - they would have to drive it for a while then get in slow traffic to see the problem. It seems from what I have read here, it could be the fan. For the previous year, in the summer, when I would stop the car or put it in park, it would make a loud whirring sound - I thought it was odd, but now am wondering if it also was fan related. Any suggestions would be appreciated.