That sort of tweaking won't get you that far, as an RWIN of 65535 isn't the best setting for your advertised speeds, and MTU can only be lowered, not increased, or you get packet fragmenting. I suggest that you use Cablenut and VanBuren's .css file for a 6000/768 Kbps connection. Comcast doesn't seem to be one of those evil ISPs, it's just that sometimes issues are forgotten, because it's a big company. Another idea would be to do a tracert to testmy.net to see if you somehow have a bad route to here.