Any ideas to why a pontiac sunfire would over heat?

Im looking to buy a 97 Pontiac sunfire but i want to see how much or what i am going to have to do to stop the over heating problem. I drove the car around a few blocks and it idled for a few minutes and the temp didn't get any higher. She says that it only starts to over heat on longer trips? thanks for the help!
The girl said she just replaced the thermostat.
 
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