Because broadband is delivered over your phone line, if your phone line has problems then your broadband will too.
OP seems focused on the internet side, but nothing has been said about the line.
Time to go back to basics and work from there.
End result for OP if released by PlusNet could be no better than it is now. If the issue is the line.
\_0-0_/ AdsL is Hell \_0-0_/
Wats SUP doc.... You using too much.....