I've noticed this behavior several times lately:
I start a HD stream, but the debugger says "can't play a 1.5 MB stream on a 1.5 MB network." So, I get only three dots.
I immediately restart the stream and it improves to four dots.
I restart again and I get four dots + HD. The title then plays through at this quality level.
It almost seems like "something" in the chain is saying "give him a slow stream and see if he's satisfied; if he complains, raise the quality a notch."
(My Internet connection is very stable and always tests out at > 5 MBPS.)