My 150Mbit Service has slowed to 25Mbits on a good day, no change to network???

I am getting 25 to 29Mbit connection on a 150Mbit plan with 5 static IP (that I have also been unable to successfully use because of the intermittant and slow nature of the connection. My Comcast supplied modem/router is Model:  DPC3939B   Vendor:  Cisco   Hardware Revision:  1.0 Processor Speed:  447.28 MHz   DRAM:  524288 MB   Flash:  128 MB My network performed flawlessly until July 2017 with about 117Mbit connection. No changes were made but post July 2017  it has just been all over the place...intermittant, and SLOW!! I am paying for at least in the ballpark of 150Mbit connection..25Mbits isn't close. I have been sending in logs of speedtests to Comcast and I get offers for Comcast Business Internet. Really????? I have done all of the recommended troubleshooting. Please advise