SN Margin increases when I turn on the phone

This this is probably like my 6th year calling their technician up to get it fixed and I was wondering why does this happens once every year? The modem would often lose connection, SN margin would go into negative numbers, and the downstream would be cut by half the advertised speed, but if I were to turn on my phone everything is all well. According to the technicians this is coming from their end so why the hell does this happens once every single year? It's so annoying :l