I'm connected to the Chiyah CO, I had similar but shorter down times even before the upgrades. I thought it was my router at first, but I don't think it is. It also may be my line :/ My attenuation isn't the best out there. But the biggest chance is a congestion, the problem always appear right after 8PM - 9PM, sometime it's for less an hour, and yesterday lasted until 1AM.
Line Attenuation (Up/Down) [dB]: 27,5 / 50,5
SN Margin (Up/Down) [dB]: 21,0 / 27,5
Edit:
The problem just appeared again, I called them, and the one who answered directly told me it's a problem from their side, and that I should just wait for them to fix it. I'm sure now it's an overload on their DSLAM or something. And what's great, that I can't do anything, and all I got from the tech guy is to "wait for a week or so"! I think I'll go buy some gasoline and burn myself....