Link light isn't anything directly to worry about. As per manual
LINKOff when no Ethernet/USB device is present,
illuminates to indicate that an Ethernet or USB device is
connected, and blinks to indicate that data is being transferred
between the PC and the cable modem
Page 13 »www.cisco.com/en/US/docs/video/a ··· 02_C.pdf
First thing I would do is bypass router and run speed tests. Looks like something is throttling your connection. Notice how the speed is high at first, and then goes down during the test? I would say there is heavy traffic somewhere else on your network which is getting priority but that's just a wild guess.