Seeing as you don't say what checks you've done I may cover things you already tried;
Disconnect all other phone devices, try another filter, try another cable from socket to router, plug the belkin directly into the test socket you can find by unscrewing the lower half of your master socket (well, if you have the correct type, if you unscrew it and find no other connector you're out of luck), borrow another router or test it on a friend's ADSL line.
Just went through 7 days of same thing on a new install with VirginMedia. Router kept disconnecting and ADSL light flashing. Eventually it would log in again. Belkin support suggested enabling "Dial on Demand" and setting Idle Time to 0, and as well I found increasing MTU to 1492 and enabling IMCP ping an BOTH router and PC got it going again. Changed filter as well. Also speed initially seemed to be less than on my modem as well, but it now seems to have stabilized now.
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