I'm no expert, but have suffered line problems in the past that have crippled my broadband.
May not be a line fault problem for you, but if it's a BT engineer coming out (now or in the future) ask him to put the BT Hawk on the line. It's not a trained bird lol, but a tester that can accurately locate a fault located somewhere along the line.
I had a BT engineer out recently due to irregular slow speeds & dropouts and his normal test routine showed nothing at fault. I was about to have a hefty call out charge on my phone bill when I asked about the 'Awk. He went and got it from the van (why he hadn't used it in the first place, I don't know) and once connected it identified a problem 30 metres away up a pole.
As I said, may not apply to your problem, but worth passing on.
Isp's so far:
4uNet, ic24, IGclick, BigBlueSky, 08002Go, V21, Biscuit, Fast4