There are many sources for that issue but the most common cause is the miswired socket. It's quite dangerous so either research it or have your electrician check it out.
Here's the small device I use to double check the sockets.
My cable and internet service provider came and informed me that they had to shut off service because I was receiving too much voltage in my coax cable lines because it was not being grounded properly. The voltage that was being shown was around 40 - 60 volts. I have two TV's and a modem for internet. I just recently replaced my outlets to GFCI outlets thinking that might help; however, they came out yesterday and informed me that the voltage was still there. I am thinking of replacing all of my surge protectors and running the cable through the protectors. Will this help or does anyone have any ideas as to what could be causing this problem? Thanks in advance for your help and suggestions.