I have Blast. The 100 mps comes in closer to 92 to 96. I use a rented modem from Comcast. The wi-fi is generally good, too.
Unless the Comcast cables to your house and then to your modem are hosed, I would suspect your modem. I hate paying rent, but I have upgraded twice in 4 years. The rental fee has gone from 8 to 11 a month, so still cheaper than buying a new one every 2 years. BTW, no router required and the wi-fi has a good reach and speed in the 40s, usually. My last modem had a good ethernet connrction, but the wi-fi degraded horribly. I worked with them on-line and whatever upgrade they did blew out an ethernet port. I just took it to the office and exchanged it. They had mailed a new one a month before,saying I was due for an upgrade when I called to question low ethernet speeds. Must have been a lemon, but the advantage of renting is insta-fix!
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hold [exit] 5 seconds...press [down arrow] twice... press  = diagnostic menu
press [exit] 3 times....type in 0030......changes the skip head (page up and down) from 5 minutes to 30 seconds
hold [setup] until light turns green then type in   ......unpairs remote from cable box(only needed if you replace your box but not remote) (setup then xfinity will pair remote)
press [exit] 3 times ....type in desired picture resolution   .....  .......    changes picture resolution. x1 comes on default 720
Hold [setup] until light goes green then hit [B] changes x1 remote IR to legacy platform IR
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