97 ford tauras won't start when hot?

My father-in-law has a 97 Ford taurus. It starts fine every morning and he drives it daily as long as it stays cool. The second the motor gets hot. It turns over,but will not start. he has to wait for it to cool down for it to start again. any ideas what it may be and how much will help alot. Thanks in advance to all who help us out.