There is very likely a poor connection somewhere on the service neutral. The cable coax is connected in parallel with the neutral as a result of the code rules. Any arcing accompanied by lights dimming or getting brighter when disconnecting a coax is a very strong indication of a neutral problem. Note that in the case of an open neutral there is also a shock hazard...you could have 120 volts between the two parts of the cable system after you disconnect them.
