@ClashOmega 

There are few things that you could check yourself:

1. Check your telephone wiring for loose joints.
2. Is the adsl splitter installed correctly and no parallel direct connections on the modem line or the jack position reversed.
3. Check for loose power adapter connections.
4. Is the modem cool? (i.e. you have not locked it in a cabinet)
5. Check your SNR levels in the router's status page - if they are too low then either splitter is faulty or the phone line is noisy.

If all these are checked and fine you should ask your ISP to check your phone line or pay visit you on site.

They should have a spare modem/router to replace and check if the issue is still present.