My Car will not start when it is hot outside. on 2003 Buick Rendezvous

When it is hot outside my car will not start. When I turn the key in the morning or late evening the car starts as if nothing is wrong. However, when it is hot outside it does not say anything, all of the gadgets are working it just will not start until it gets cooler in the car. I'm thinking I have an ignition switch or computer problem. What is my next step.