98 Ford Escort Zx2 overheating problems?

Wednesday I drove to the college world series (I live in Omaha) and it was about 95 degrees. I was idling in traffic for a long time on the interstate while driving down there, and then again downtown. The engine started steaming, there was no damage to any of the hoses when I popped the hood. I was low on coolant, so I filled it up and had no problem getting home in it that night.

Yesterday was only about 80 degrees and I had no problems driving it all day, even sitting in rush hour traffic, temp gauge was perfect.

And then today comes, it was about 90 degrees today, and I drove around to test it out with no problems, until I pulled into the drive thru, I waited for not even 5 minutes and my temp gauge was at 3/4ths, but as soon as I left the drive thru and got to going 45-50 mph it dropped back to normal in an amount of seconds.

I believe it is either the thermostat or the fan, but I could be wrong. Does anyone else have any ideas?

The grills on cars are so small these days that there isn't good airflow when stopped, idling, or sitting in traffic. I don't think the thermostat is the problem.
Back in "the day" when engines faced forward, the fans were on the front of the engine and either ran all the time, or (later) had a coil on it that when it got warm, the coil expanded and turned the fan.
These days, the fans to cool the engines are electric. The "thermistor" is supposed to engage when the temps come up. These fail on a regular basis. The car will stay fine and cool when it's moving because of the air flow through the front of the car.
But, when stopped or idling, if the electric fan doesn't turn on, the engine will overheat.
First, I'd open the hood and find the fan. Get a piece of paper and hang it between the fan and the engine. Turn the car on and let it get warm,. Watch the temp gauge for it to get to "normal." Then go watch the paper, Does it blow up against the engine or sit still? If the fans run, then the paper should react to the wind.
If the paper just hangs there, then the fans aren't coming on.
That's on a fuse which you could check first but it could be the fan control, thermistor that isn't turning the fans on.
If you can SEE the fans you can watch them rather than using the hanging paper, but these fans are behind shrouds and often hard to see, especially to see if they are spinning.

Coolant system may need to be flushed. I would definitely change the thermostat regardless and see what happens because its such an easy thing to do.