There is your problem, .5 mbps (it is megabits) is not enough. I just ran a set top box test and have my normal average of 2.99 mbps. I have no buffering problems. I don't trust the on-line speed tests, I have 6 mbps service from Comcast but the on-line speed test has shown 12 mbps and higher.
Could comcast be throttling the bandwidth somehow? They could be purposely targeting the Sky Angel IP in the case of this customer. Just a thought?