Since you are using cable modem SB8200 if the internet is dropping in and out you to check for the cable signal level and also need to check for the LED lights on the modem when internet drops. If you have the router connected to the modem please do connect the device directly to the modem and monitor connection and also check for the signal level by following the steps mentioned on the below article.

http://arris.force.com/consumers/articles/General_FAQs/SB8200-Cable-Signal-Levels

Nothing helped, bought another, more expensive brand and no longer have any problems 

If you are experiencing with intermittent internet connection, usually if the cable signals are not in range.

http://arris.force.com/consumers/articles/General_FAQs/SB8200-Cable-Signal-Levels