I had the same problem too! After fighting with Comcast for 90 days. The 8th tech was sent to try to fix the problem. He did one thing all the others failed to do. He took my cable modem to the outside main connection point. Used a Power supply from his truck and called his office to ping my modem. He informed me the settings were wrong and needed to be adjusted at the node. Also, I had too many splitters before the modem as well. He said I needed to have a custom wiring to the PC modem.
After the tech left within 24 hrs, Comcast made the adjustment on the node. Yea!
Before the adjustment I was slower than a 28.8 modem and now I
