MacBook Air :: CPU Shutdown (Overheating) - No Periodic Activity On Graph

I have a new wk 11 MBA 1.6/80g. I've been reading threads on CPU heat and core shutdown. I'm trying to decide whether I need to return my sys for an exchange. Other than heat/shutdown issues my MBA is awesome. Excellent fit and finish, no dead pixels, lid alignment is perfect, etc. When playing HD vid at full screen with my desktop background randomly changing at 5 sec intervals the vid stuttered some and I saw one of my cores intermittently shutting down. However, I learned to stop the stuttering by making my desktop background static. I was thrilled. I could overload the system with two vids playing simultaneously and watch my core temp go to 84/85C with some stuttering on the vids and some intermittent core shutdown. Finally the temp settled to mid 70's. However, I later noticed that I was having periodic core shutdown at 69C. This seems way low for a core to be shutting down.

Ambient temp for my room has consistently been 70F through all of my tests. As I type this message my core usage is very low, temp is 47C. My activity graph shows intermittent periods of both cores having no activity. How can the graph show no periodic activity on each core while I type this? I thought core shutdown was when the core shows no activity for some period of time. Perhaps periods of inactivity which really indicate core shutdown could be different as compared to very low demands on the CPU's? But, am I misreading core shutdown? Is my unit defective? Is excellent fit and finish/build quality the norm for MBA? I don't want to exchange and have dead pixels or a warped lid, etc. I would rather open the unit and mess with thermal paste but I can't see it correcting core shutdown at low temps under heavy load. I purchased this MBA on Saturday.