A few questions:
What speed do you normally get?
What model of modem (or modem-router) do you have?
Does "ADSL" mean multi-mode / auto, that is "auto-select from adsl2+ / adsl2 / adsl1=g.dmt" ?
Sounds like a line fault. I have had something vaguely like this a couple of times, cycling, not sure what caused it though, it was something to do with me fiddling around at my end although I don't expect that applies to you.
Call your isp straight away.