It seems like on warmer days it will not turn over?

I have a 1999 Honda Accord with about 95,000 miles on it. The car is an automatic and it is in fantastic shape but sometimes I have troubles starting it. It seems like on warmer days it will not turn over, I will try it for a couple minutes and the car sounds right it just won't fire up. I will come back 20 minutes later, try it again and it will start right up. This problem is occuring more and more frequently. I have taken it to two different Honda dealerships and they have run tests on it and can find nothing wrong. They say it may be a computer chip malfunctioning or something but no one can give me a definite answer since my car conveniently never acts up when it gets looked at by a mechanic. I also have had the timing belt and spark plugs replaced recently and still have this occuring. Have you heard of anything like this? It seems to have everyone stumped!