If your car is turning over normally, then you can rule out the starter.
From there you need to do a little troubleshooting.
When it's warm and won't start, does it cough and sputter or just crank over?
If it just cranks without trying to start at all, does your upshift light go out while cranking? Do you get a tach reading while cranking?
If it sputters and coughs while trying to start warm, is your tach jumping erratically? Does your fan turn on when the temp gage hits the "M" in "NORM"?
There are quite a few reasons you can get a hard start most of them are low cost sensors ($30-$60) but finding the problem can be a pain.
Aaron