Modem connection problems

I have a SB4200 SURFboard Cable Modem and my internet will slow down a lot of just lose connection. I know it has to be the cable modem because the online light goes out. I have talk to Comcast several times about it. Each time they tell me to unplug the modem wait awhile then plug it back in or try to tell me it is because of my d-Link (but why would the online light go out if the modem is connected to the line then send the signal to the d-link plus I can access my other pc though the d-link just get go online). Sometimes this will stop the problem and sometimes it will work for a little bit. They keep check my modem but every time they visit is is working normally. Is there any way I can fix what ever is happening?