download this istat pro from
http://www.islayer.com/index.php?op=item&id=7
installed and after that go to system preferences, under istat menus, click CPU on your left, click show CPU in menu bar...then you will see both of your cores processing on the top...you might want to change the settings to "update every 0.5s" so that you can a better picture of your cores processing...
try play youtubes more than 5 minutes full screen mode...the temperature will go up...and then at about 70 C , your mac will freezes then unfreezes then freezes again...due to one of the core shutting down...
the theory i believe after reading so much is the temperature dissipation causing core shut down.
and temperature dissipation not good might be caused by fan not strong enough, design not good, thermal paste too much.
I have not tried, but maybe running macbook air in a freezer compartment maybe will not cause this problem. you never know. There is no standard test here, some users say they run very smoothly without core shutdown, maybe they stay in cold countries. I from Singapore, temperature maybe slightly higher here.