. I've been away from the boards for the most part in the last week, I didn't see anything asked me anything.
Actually, the arc reactor glows and moves the more that he needs it, remember it doesn't power him, it keeps the shrapnel from his chest. In Iron Man 1 it light up when it was first put on and was seen on in the video surveillance, it then was "dormant" really after that until the 2.0 scene, some scenes on and some scenes off.
Iron Man 1:
Iron Man 2:
The leading thought is that as he exerts himself, the shrapnel would be pushing forward and so the arc would kick on to pull it or keep it back in place. If you watch the film as Tony gets upset or angry it lights up because the quickening of his pulse and heartrate would cause the shrapnel to continue to push towards his heart.
As he was sitting around the table after Coulson died, he was going through a range of emotions so the change in his heartrates, etc it would continuously go on and off, blink if you will. Even during the Loki scene it's off and on because Tony is unsuited facing a god and trying to keep his cool, it would be trying to counteract his body changes which could send the pieces into his heart and kill him.
Theoretically as he has sex for example, his arc should blink and glow in different intensities because of all of the exertions and the heart rate changes.