>>> instead of continuous has an intermittent tone with approx one second bursts
Are you sure that isn't just the 'message waiting' tone, which is a two-tone sound (a bit like the old police sirens but slower). If it is, call 1571 to retrieve the message. Then see if things have got better when the dial tone returns to normal.
Also be aware that ADSL splitters can pack up after time, resulting in connection problems. (Written from experience; I've gone through several over the years!)