permamac,
Are you able to play full hd videos on this MBA? That would clearly indicate something.
Today is cold in here, so I just watched a 7 minute flash video on gamespot.com (which is higher resolution than youtube videos) and it behaved just fine. Although the fans started to spin higher as the cpu usage grown, they never reached more than 5400 rpm, and 2 minutes after the video playback stopped, they went back to 2400 rpm.
Also, during the playback, both cores remained active and never reached 100% (although they neared for a brief moment). Temps got up to 80º C, but they kept an average of 71º most of the time. After playback, they dropped to 57º.
And yes, I was using the external 1680x1050 display. Also, I never had issues after connecting.
My doubt is abou playing full hd videos. I never had a problem on my previous macbook.