This is an odd issue that I have witnessed 1st hand a good mate of mine uses ethernet power line adapters for his Desktop PC, visited him to-day and the first thing he says my broadband is not working its working fine on the wi-fi laptops so my attention is fixed on the desktop PC using power line adapters the network is showing 100 mega bits per second but no access to the internet.
I ask him have you made any changes around the house he said I got Fish Tank led lights, so ask him to turn them off and straight away the desktop could access the internet, told him to turn them on again but no access to internet.
I am just putting this odd case into the Kitz forum incase others come across the same problem.