Wont start when engine hot

My 1996 saturn sw1,112000 miles wont start when engine is hot but starts and runs fine when I first start it in morning. Starts normally and runs fine untill I turn off ignition. Then it won't restart for about an hour. All electrical operates normally, radio. Lights. Etc. When key is at start posistion engine will crank, but wont restart. It seems that something needs to cool down. Then it will restart and operate normally untill I turn it off.
What can cause this problem? I have spark and replaced coil packs, what else can I try?