Did they ever sort out your problem? I've had to cancel the service and switch suppliers. Maybe they've overstretched themselves and can't fix the problem at all. I had a box change, cables changed, work done outside, work done inside to no avail.
I think there's something they're probably not letting on with this. Had no issues for years and then terrible 6 months. Maybe it's the upgrade of broadband speeds? Either way they didn't seem to concerned about me leaving or try to get me to stay which indicates more hers than meets the eye!