•The cable modem itself is faulty.
•Mains wiring to the cable modem is faulty or intermittent.
•Mains spikes arising from e.g. vacuum cleaners, power tools, heating thermostats, refrigerator thermostats.
•Electromagnetic Interference (EMI), e.g. mobile phone, local radio transmitter.
There are plenty of reports of mobile phones disrupting cable modems if kept too close.
If faulty cable modem, you can take a trip down to your local Road Runner/TWC - Brighthouse Center. Locations listed here.