For no obvious reason, but more often at weekends, my ADSL light on my router flashes and I just cannot get onto the net. Running a diagnostic on the router shows an ADSL connection and ADSL synchronisation issue. I'm just a thick pensioner and this means very little to me. Is it the phone line or what?
This means there is either a line fault between you and your exchange or the exchange itself, unfortuantly it is a fault which Plusnet can do very little about, you will need to raise a ticket with Plusnet explaining that you have a sync fault on your line, they will in turn contact BT who will eventually get round to fixing it, I stress eventually because they are rarely in any rush to fix problems.
Just thinking of some tests you could try to make sure its at the BT side of things and not with you (if the fault lies after your master box and BT come out they will charge you £50) Have you got DSL filters on all the sockets around your home?, has any wiring changed recently?