|Check the incoming connections. If a phone line cct incoming, check the actual point of entry into your home; verify it's sound. Verify there are no breaks/faulty joints in the incoming path.|
Change the filter if you use one.
If you're router is not plugged into the master socket, then plug it in there and see how thing are?
Also verify master socket connection is sound (though if it was until recently there ought to no reason for it to change...?). Master sockets have been known to fail; as have phone line filters.
If your using cable as your isp source... Again verify incoming cable, connections too; and any terminations/master sockets/filters if involved/present.
And in both situations double check for "any" in-line connection in the actual incoming cable (be it phone or cable service).
When I had intermittent dsl service it turned out to be corrosion on at least two of three joints in the incoming phone cable. The cable had been extended twice around the house exterior, and both extender connections had degraded significantly. A new
cable incoming with no breaks/in-line connections resolved it all.
Although CurtR has taken you through router checks, perhaps borrow another router to test the cct.?
Similarly it may be worth leaning hard(er) on your isp to get the incoming physical line/cable fully tested, which means they come on-site and test the cct. from your end. There is a series if simple test that are performed on-site with proper test kit. Those tests will, if properly conducted, verify the state of the incoming physical connection - from the street cabinet to your home); impedance of the path; signal strength. Possibly the incoming signal to noise ratio too; and if this way off kilter it will seriously impact on signal stability...