I've had 3 Comcast technicians come out to my house to figure this out and no one can fix it.
I get fluxuating speeds from 6-18mbps with 12 mbps being the average. I'm in the Blast package and am barely getting Performance speeds. My upload speeds are great at 5mbps. I've been on the phone with Comcast for hours, days and weeks and the only advise I get is to send a "Refresh" signal or "Plug/Unplug" solutions which do nothing. I've gone through 2 modems/router combos thinking that was the problem. The last service tech told me to downgrade to the performance package since I can't get Blast <Seriously>. I'm tired of watching movies on Netflix and Amazon only to have it freeze and rebuffer 5 times throughout the movie. I have the newest modem as well (Technicolor TC8305C) and the boot file has been checked and verified to be Blast multiple times. I can't wait for Verizon FIOS to get into this area.