My poor old mum has had this buck passing from Talk Talk & AOL. It's not us it's your end. Last time it happened, I went round, took a look & was able to prove it was TT's modem at fault, so I rang & complained.
They denied it was their fault, I presented them with incontrovertible evidence, they still denied it. I wrote to Charles Dunstone three times a day, until he responded, well his PA actually, calling his team amongst other things an absolute shower and muttering about Watchdog & now mum has two years of free broadband, phone & a nice bunch of flowers.
I hate the buck passing - go directly to the top man & persist.