Flash tests are not that great but neither is that site you used.
I get 36/5.5 (with PB) on my blast on speedtest.net and comcast speed test, but that site gave me 10/4 lol
That said, youtube is sometimes just slow. Videos don't steam with much more than a few Mb so if you got over 5 you can get HD fine. It's probably routing or server issues with them. Some videos stream slower vs others also as are cached differently.
that said, your upload on Shaperprobe looks low, but download is good. Try a few more tests with it. sometimes it grabs a far server too. Also see what the comcast speedtests show. If they show poor upload you have an easy way to complain to comcast to fix