1991 Honda Accord car will not start

This has been an on going problem for about 2 years. It started this problem on occasion, sometimes months apart. Recently the problem has become more frequent. It occurs mostly when the weather is very warm and after the vehicle has been run for awhile, although on occasion it wont start when cold. When I say wont start, the engine turns over but will not fire. When this happens if we wait 20 minutes, sometimes an hour or so the car will start like there was never an issue and run fine. There may not be another issue like this for several days or a week. Then the same problem again. Hope you can help, Thanks.