1999 Honda Accord Summer heat preventing engine start up?

I have this problem when I park my car outside under the sun in the summer time for extended periods of time (4-8 hrs). The engine will crank, but it will not completely turn over and turn on. What's weird is that the last few times this has happened, I did a couple of non mechanical things that worked. I lifted the hood for a few minutes, and I rocked the car back and forth from the back of the car. Sure enough, the car turned on. My gas tank has been half or full so I know its not lack of gas. I learned that when I park in the shade I don't have this problem. While the shade is a short term fix, I would like to understand what the problem is and fix it for good. Any ideas what could be causing this weird problem?