I had this problem for months and months it was frustrating as hell. Turned out my micro filter must have been damaged as soon as I changed that the problem stopped happening, but before I did that I tried all sorts of stuff from resetting the router, releasing and renewing IP address, checking my internet settings, all with limited success. You might call the micro filter something different where you live, its the little junction box that the cables from the router and other devices plug into before it goes into the phone line.