2003 CL won't start when engine is hot on hot days

I have this strange problem for a dozen times this summer. In the mornings, my car always started fine. But when I turn off a hot engine, it won't re-start. I had to pop hood every time and wait for 10-20 minutes, then I re-try and it would start fine. My battery is less than 2 yrs old, I think battery is ok.

Anyone had this problem too? what's the cause and fix? Thanks a lot!!