I get the same thing happening to me if I'm stuck idleing for more then a few minutes on a very hot day. (35+)
Normally goes away after moving for a minute or so though, once some air starts to circulate through the engine bay.
I've recently raised the rear of my bonnet to try and reduce the engine bay temperatures, but haven't had a hot day since doing it, so can't give you any findings.
Can definately see some 'heat haze' coming out near my windscreen now, so obviously some of the heat is escaping.