Could be clogged injectors. I don't have a car with injectors, but my sister swears by injector cleaner that you pour into your gas tank. It won't hurt to try it.
When the temperature gets hot outside, there's fewer air molecules per square inch of air. That means the air entering your engine is less dense. If you don't have enough fuel entering into the engine, the decrease in air density in the combustion chamber makes it even more difficult for the decreased amount of fuel to ignite.
Get a tune up to eliminate potential ignition issues, put some injector cleaner in your tank and see if your problem goes away.
If not, you'll may need to spend the bucks to get it diagnosed.
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