Is there a diagnsotic that displays the internal temperature of various sensors inside 27" iMac late 2009?

I believe I have a thermal problem which on warm days is the cause of the display to black out many times. Apple checked the iMac in their cool workshop and the fault has not surfaced. The computer is working, the sound up/down keys make their bleeping sound, the external secondary display is working and time machine is backing up OK, emails are received but the main screen is black. Have to wait about 30 minutes (sometimes more and sometimes less) and eventually pressing a key will fire it up again. Is it a thermal problem or possibly an intermittent hardware that might leave a log for the reason the screen blacks out? Thanks