What modem do you have?
next time you speak to them ask them what your Downstream SNR is. It should be >30db.
If you have a Terayon Terajet modem I would bet that your SNR is low and the modem is dropping in and out of service.
They should come and sort out your install and get your SNR >33dB, but they will probably just change out your modem for a more resilient model.