I've experienced some netflix buffering issues on my shield as well. In my experience it is simply just the wireless connection strength. I have about 50% connection in my basement and speed tests and most things appear perfect (Can stream perfectly too) but netflix will buffer occasionally even though it seems like it shouldn't. Probably just loosing some packets every now and then cause it doesn't happen when in a stronger connection.
As for the screen dimming and brightness this is a known issue and posted heavily about in the main nvidia forums. I'm not sure if the cause is quite known yet and i see it usually when web surfing but it doesnt really bother me much, but would certainly be happy if a software fix is released that fixes that.